The Social Group or Team Leonard refers to the group of best friends consisting of the main characters of The Big Bang Theory. This term is first used to described the group of best friends by Sheldon in "The Panty Piñata Polarization" (EP. 2.07) although Leonard has used the term previously in "The Middle Earth Paradigm" (1.06) to describe Penny's friends.
The social group or Team Leonard was formed when Leonard befriended Howard and Raj, as told in retrospect in "The Staircase Implementation" (3.22). As Sheldon was living with Leonard, Sheldon constantly butted in when the three were hanging out and he quickly became a member of the group. He later established himself as the group's leader and comic relief although no one agreed that he was either.
Penny joined the group after moving next to Sheldon and Leonard in "Pilot". She was "officially" recognized by Sheldon as a permanent member of the social group in "The Panty Piñata Polarization" right before she was banished briefly for unwilling to move away from Sheldon's Spot. Or when Penny officially acknowledged that they were friends in "The Euclid Alternative. Bernadette joined shortly after she started dating Howard in "The Gorilla Experiment" (3.10). Amy started hanging out with the group in "The Zazzy Substitution" (4.03), and she mentioned officially being part of the group in "The 21-Second Excitation" (4.08). The three girls formed their own social group and have frequently been hanging out together as Penny's Posse.
Raj's sister Priya started hanging out with everyone in the group after she had developed a relationship with Leonard during her stay in Los Angeles, as seen in "The Cohabitation Formulation" (4.16). The other girls actually formed a Priya-hating group behind her back in the beginning as she and Penny did not get along because of Penny being Leonard's ex-. They began to accept each other eventually though shortly thereafter Priya returned to India and was out of the group.
Sheldon once "befriended" Kripke so as to use the new Open Science Grid Computer that Kripke controlled. He had to kick Raj from his friends because he could not maintain five friendships. He ousted Kripke and got Raj back shortly afterwards when Kripke told him time on the computer followed an official schedule instead.
Amy forced Sheldon to realize that Leonard was the true nucleus of the group instead of him in "The Toast Derivation" (4.17). As a result, Sheldon tried to create a new social group with himself as the leader. He invited Stuart, Kripke, Zack and TV's LeVar Burton over to his apartment for a good time. Burton did not show up (until very late). Sheldon abandoned his new friends when "they were having fun wrong".
- Leonard Leakey Hofstadter - (The leader of the group) the nucleus, muscle and Kirk. (Captain James T. Kirk).
- Sheldon Lee Cooper - the whimsical elf (Is the main antagonist and invisible member of the group. Co-leader of the group which he is not in his mind) Things revolve around him and he often makes many choices for the group), smart one and Spock.
- Howard Joel Wolowitz - the funny one and second in command of the group. and Scotty (Montgomery Scott).
- Rajesh Ramayan Koothrappali - the group historian, lovable foreigner and "the guy who always gets killed". or the Star Trek guy in the red shirt.
- Penny - as of "Pilot" (1.01).
- Bernadette Maryann Rostenkowski-Wolowitz - as of "The Gorilla Experiment" (3.10; Started hanging out with the gang) "The Hot Troll Deviation" (4.04; Became a main cast member).
- Amy Farrah Fowler - as of "The Zazzy Substitution" (4.03; Started hanging out with the gang) and "The 21-Second Excitation" (4.08; Became a main cast member).
- Stuart David Bloom - as of "The Date Night Variable" (6.01) until the "The Bakersfield Expedition" (6.13) and once again as of "The Locomotion Interruption" (S8E1). Also he was once invited by Sheldon to form a new social group in "The Toast Derivation" (4.17). Before joining the group, he could have been considered a very close associate.
- Denise joined the group in Season 12 when she started dating Stuart.
- Bert Kibbler is an honorary member of the group.
- Wil Wheaton is also an honorary member of the group and also an ally of them. Wil is well-liked by everyone by the team, but he and Sheldon have an on again and off again friendship.
- Leslie Winkle - used to join the guys at activities on occasions (main cast member from "The Codpiece Topology" (2.02) until "The Cushion Saturation" (2.16)).
- Stephanie Barnett - not counting her boyfriend Leonard, only hung out with Sheldon, though he said she was only "tolerable". But then he compared her to Dr. McCoy in Star Trek. He did like to have a medical doctor around to keep checking up on his hypochondria.
- Barry Kripke - befriended by Sheldon for his wish of using the new Open Science Grid Computer; got kicked out when Sheldon found out he was no use to his cause. Barry is still an honorary member of the group and ally.
- Zack Johnson - once joined the guys to dress as the Justice League and entered a costume contest held by the comic book store in "The Justice League Recombination" (4.11). Zack is still an honorary member of the group and an ally.
- Barry and Zack, along with Stuart, were members of Sheldon's failed social group, created to be independent of Leonard as the nucleus, as seen in "The Toast Derivation" (4.17).
- LeVar Burton also visited the occasion because Sheldon tweeted him, but he quickly left after seeing Sheldon's new friends singing karaoke, one of them (Stuart) only wrapped in a towel.
- Priya Koothrappali - joined the group during her stay in Los Angeles while dating Leonard during season 4.
- Emily Sweeney - main character from "The 2003 Approximation" (9.04) to "The Application Deterioration" (9.18), though she seldom takes part in any gatherings and is no longer in it by Season 10. She shares a mutual hatred with Penny.
- Anu - Raj's former fiancée and ex-girlfriend. They broke up in "The Maternal Conclusion".
Teaming for the Physics Bowl: The guys enter the university Physics Bowl as a team in "The Bat Jar Conjecture" (1.13). In a simulated run, Sheldon ignores the others' buzzes and keeps answering for them, leading to the three kicking him off the team. Sheldon goes on to form his own team with some common staff from the university, and the three get Leslie Winkle to be their fourth team member. Sheldon's team loses when he does not allow one of his teammates, who was once a physicist in the former Soviet Union, to answer the final question.
DEFCON 5: The guys come to Howard's rescue when he tried to impress a girl, Stephanie Barnett, by taking her to a NASA facility and letting her drive the Mars rover, but has got the rover stuck in a ditch instead, as seen in "The Lizard-Spock Expansion" (2.08). Leonard is asked to send Stephanie home (and they end up making out and starting a brief relationship). Raj and Sheldon try to help Howard to save the rover but fail. Then they destroy all the evidence and run. NASA then announces that evidence of life has been found on Mars in the ditch he got the rover stuck in and can't claim credit for it.
Woboto-a-woboto: The guys make a killer robot, the Mobile Omnidirectional Neutralization and Termination Eradicator, or MONTE, to enter the Southern California Robot Fighting League Round Robin Invitational in "The Killer Robot Instability" (2.12). But they are challenged by Kripke to fight his killer robot, the Kripke Krippler. MONTE did not survived the fight.
Penny Blossoms: Penny starts a business of making and selling flower barrettes in "The Work Song Nanocluster" (2.18). Sheldon and later the other three help her with production and setting up a website.
The North Pole expedition: Sheldon is offered to go on a National Science Foundation expedition to the magnetic North Pole to detect slow-moving monopoles in order to confirm string theory in "The Monopolar Expedition" (2.23). The other three join him as his support team. They return three months later in "The Electric Can Opener Fluctuation" (3.01), and Howard tells Sheldon that they tricked him into believing he had found the magnetic monopoles by creating static from turning on and off an electric can opener because he acted like a "dictator" the first few weeks when they found nothing. Sheldon has already sent an email to everyone at the university announcing his success. He has to send a retraction email to set the record straight. His reputation is severely damaged as a result.
Speech for Chancellor's Award for Science: Sheldon is afraid to give a speech at the coming Chancellor's Award for Science in "The Pants Alternative" (3.18). Raj, Penny and Leonard come together to help him overcome his stage fright. Howard just stands by with his superpower of "the ability to pretend like I give a damn".
The bowling matches: Sheldon leads the guys and Penny against Stuart's team in two bowling matches in "The Wheaton Recurrence" (3.19). Sheldon's team loses the first match after Penny breaks away over a fight with Leonard about her not returning Leonard's "I love you" the previous night. They have a second match again and this time there is a bet. Sheldon's team loses again after Wil Wheaton from Stuart's team sways Penny into breaking up with Leonard. The four guys dress as superheroines and a supervillainess in the comic book store for losing the bet.
New Year's Eve costume party: The guys invite Zack and Penny to join them dressed as the Justice League of America in the comic book store's New Year's Eve costume party in "The Justice League Recombination" (4.11). Penny nearly backs out, but Leonard finally talks her into going together. They win "Best Group Costume" in the party.
The Lenwoloppali Differential Equation Scanner: Leonard comes up with an idea for a handwriting-recognition differential equation solving app in "The Bus Pants Utilization" (4.12). The 4 guys form a team to write the app, with Leonard as the leader. Sheldon keeps trying to steal the leadership and proves to be impossible to work with, and is quickly fired from the team. Sheldon is later persuaded by Penny to apologize (sarcastically) to the guys and is accepted back to the team. Then he goes back to his previous behavior and is fired a second time. He ends up working for Penny for her "Project Shoe".
Attending a science conference: The guys, Amy and Bernadette have all been invited to speak in an annual science conference (The Institute of Interdisciplinary Studies' Symposium on the Impact of Current Scientific Research on Societal Interactions) in "The Love Car Displacement" (4.13). Amy invites Penny to come along. As things have happened during their car trip and the night at the hotel, half of them are not in a good mood when they host their talk on "Science and Society". A number of personal issues get into the talk and Penny leaves early for home with someone else. The rest drive home later, not without some trouble.
Fund-raising for the university: President Siebert orders the guys to attend a fund-raiser in "The Benefactor Factor" (4.15). One of the wealthy donors, Mrs. Latham, invites Leonard to dinner to talk about his research and hits on him. Leonard feels uncomfortable, but Sheldon encourages Leonard to go along with it to get millions of dollars for the department. She manages to seduce him in the end. He is applauded by his colleagues when he enters the cafeteria of the university.
Getting Sheldon's WoW stuff back: Someone hacked Sheldon's World of Warcraft account and stole all his items and gold within in "The Zarnecki Incursion" (4.19). The guys help him track down the thief and they go to the thief's house by car to get his virtual stuff back. The thief does not comply and the guys return empty-handed. But then Leonard's car breaks down. They get Penny to come and take them home, but she helps them get Sheldon's stuff back from the thief in the end.
Leonard and Penny reunite: Leonard (having broken up with Priya) decides to ask Penny out on a date and envisions a fantasy, in which things do not go well the second time around, but he decides to go ahead and ask her out anyway, Penny agrees to this and they restart their relationship again.
- Sheldon calls he and his best friends a "homogenous group" while he fires Raj from his friendship in "The Friendship Algorithm" (2.13).
- Bernadette is the only group member not to appear on or get involved in "Sheldon Cooper presents Fun with Flags"
- Bernadette had been the only group member not to meet Wil Wheaton until the episode "The Celebration Experimentation" (S9E17) whereas only before then, only Penny, Amy and the men had met him so far.
- Bernadette is probably the only group member not to be involved with any of the "Star Trek" franchise (nor any of the "Star Wars" franchise) and whereas only Penny, Amy and one or two of the men are involved with some of that sci-fi stuff to which is why Bernadette got very cross with Sheldon for doing a wedding vow in Klingon and even more crosser for one last second when Sheldon had said "The Klingon High Council" during the rooftop wedding of Howard and Bernadette in "The Countdown Reflection" (S5E24).
- Leonard being group leader might reflect that Leonard is one of two main characters in the show, and Sheldon being the 'antagonist' of the group may reflect him being the second.
- Stuart is the only member who has yet to make an appearance in "Young Sheldon". By that time, he would've been a teenager.
- All of the members of the social group, current or former, have the "y" sound at the end of their name, whether it be by their actual name or nickname (Shelly, Lenny, Penny, Howie, Koothrappali, Amy, Bernie, Stewie, and even Emily and Leslie).
- All of the main 8 members of the Social Group (except Penny, but she later became a Hofstadter) have the letter O in their surnames (Sheldon Lee Cooper, Leonard Leakey Hofstadter, Howard Joel Wolowitz, Rajesh Ramayan Koothrappali, Penny Hofstadter, Bernadette Rostenkowski-Wolowitz, Amy Farrah Fowler, Stuart Bloom).
- This is a trait also shared by the Pokémon series, namely with every region from the core series bearing a name with an O in it (except Galar from Pokémon Sword and Shield).
- as described in the conversation between Sheldon and Amy in the bar in "The Toast Derivation" (4.17).
- As described by Sheldon in "The Precious Fragmentation" (3.17) when the guys are going up the stairs with a box of stuff from a garage sale.
- As compared to the Star Trek character by Sheldon in "The White Asparagus Triangulation" (2.09) when he is crashing Leonard's date with Stephanie at home.
- As referred by Leonard in "The Hawking Excitation" (5.21).