Penny's Posse is the subgroup of a few of the best friends of the Social Group made up of the female members, Penny, Bernadette and Amy. They are the female halves of the relationships that have developed for the male members, Leonard, Sheldon, Howard and Raj, though Raj's girlfriend Emily didn't hangout with just the girls. The original show dynamics were how the new girl Penny affects the lives of the male members and specifically Leonard's interest and pursuit of Penny.
During season 4, Howard established Bernadette as his permanent girlfriend and Amy became Sheldon's friend who was a girl, but not his girlfriend the Posse began. Bernadette was friends with Penny from working at The Cheesecake Factory together and Amy pushed herself toward Penny claiming her as her best and only friend. Their first night of hanging out together was in "The 21-Second Excitation" while the guys were waiting in line for a re-release of "Raiders of the Lost Ark". Amy was awkward with her interactions with the other two and creeped them out a bit, but gradually they accepted each other and became friends. Penny introduced Amy to new experiences like shopping
together, alcohol, trash talking and the emotional aspects of male-female and female-female relationships. The latter experience changed her relationship with Sheldon from her acting just like him to adding experiencing sexual arousal, hand holding, becoming a girlfriend, kissing, Amy and Sheldon telling them that they loved each othe and finally being married. Another strong influence in the Posse was Bernadette choosing Amy for the maid of honor at her wedding after she was hurt when they didn't go dress shopping together. Amy is not against suggesting things that she reads off the internet that women friends do or comforting Penny in a crisis and using her as a subject in her experiments. She also has a limited attraction to Penny and thinks that Bernadette is the least cool of the Posse; however, by seasons 7 and 8 she has became closer with Bernadette since they are fellow scientists. Using the Posse, some episodes have the story lines split down the middle between males and females as they guys follow some geeky pursuit as they have all their lives and the girls hang out in Penny's apartment or have a girls' nights out. The story lines have become much more complicated with parallel plots after the additional of new cast members. They also play hooky and go to Disneyland together and get makeovers as Disney princesses. Most of the time they are hanging out in Penny's apartment, but they have gone to bars, restaurants, a tea parlor, strip clubs and a trip to Las Vegas together. The girls have a habit of helping both Sheldon with problems with life and Raj and his love-life after he loses his selective mutism.
The group goes to Vegas together in season 8 where Amy and Bernie party and get drunk. They also dig up embarrassing moments from their pasts on each other (Amy came up with the idea to watch Penny's movie: Serial Ape-ist 2: Monkey See, Monkey Kill, Penny discovered a video of Bernadette's beauty pageant days and Bernadette blurts out Amy's secret about writing "Little House on the Prairie" fan-fiction about herself and Sheldon).
Near Amy's birthday in season 9, the other girls spill Sheldon's gift to Amy that he plans to sleep with her. Amy runs out to get waxed.
In season 10, they take new mother Bernadette out for dancing and go no where wondering if they are trying to recapture their youth. Finally Amy and Penny get depressed because Bernadette has move on further with the marriage than they have (House & Baby).
In the final episode, the Posse is together for the last time learning about Penny being pregnant. Amy is shocked Penny didn't tell her since she was her best friend. Bernadette said the same thing which Amy wanted to discuss with Bernie later about being Penny's best friend.
- The only episode in the entire show that all three members of the group (Penny, Bernadette and Amy) did not get featured in for any appearances was the Season 4 episode of "The Irish Pub Formulation" (S4E6).
- Bernadette and Amy are the only members of the group that did not get dressed up as a comic superhero as of them not featuring in the episode of "The Justice League Recombination" (S4E11) to which were not required for the story-lines in that episode.
- Penny is the only member of the group to have her belly button seen on-camera and seen in a bikini while Bernadette and Amy are the only members of the group that haven't been seen in a bikini or have their belly buttons seen on-camera because they were too hung over to get off the bed and come to the pool with Penny at the end of "The Focus Attenuation" (S8E5) whilst vacationing in Vegas.
- Whereas some episodes with different numerous non-Big Bang Theory actress doing bikini scenes (like sunbathing on a balcony etc.) and having a belly button seen on-camera were usually shown in the beach house by the sea comedy show with Charlie Sheen (aka Charlie Harper who has a lot of girls) called "Two and a Half Men" (which was also by Chuck Lorre productions). Penny also had her belly-button exposed when she was doing yoga with a very pregnant Bernadette, who was jealous of her flat belly.
- Every member of the group has been topless though not seen in some episodes.
- Amy has been seen topless for the first time in bed with Sheldon and with her glasses off in "The Opening Night Excitation" (S9E11).
- Bernadette had been topless in bed with Howard in two scenes of "The Cohabitation Formulation" (S4E16), the ending scene of "The Expedition Approximation" (S8E6) and a scene of "The Sales Call Sublimation" (S9E12). She was also seen with her glasses off while she was topless in bed as well.
- Penny had been seen topless in numerous episode scenes (like showering, having sex with Leonard and so on) in the entire show.
- Bernadette and Amy are the only members of the group not to be seen in their underwear since the time Amy accidentally filmed Penny in her underwear during the last shot of the ending scene with wedding dress fitting in the episode of "The Isolation Permutation" (S5E8) and a bit of her underwear could be seen under her nightie in "The Raiders Minimization" (S7E4).
- In terms of the posse, two out of three members possess traits/habits that one of them doesn't:
- Penny is the only member who is not an intellect, doesn't have a PhD, doesn't have a career in science, isn't from California, and isn't bespectacled (not including the one instance she wore glasses at the end of "The Egg Salad Equivalency").
- Amy is the only member who has not come across as a bully, isn't a blond, works at Caltech with the guys, has won a Nobel Prize, hasn't previously worked at the Cheesecake Factory, didn't take her husband's last name, and still has yet to embrace motherhood.
- Bernadette is the only member who didn't have a shorter haircut, isn't married to someone with a PhD, lives in a house, and presumably doesn't have a birthday in December (Penny was born on December 2 and Amy was born on December 17).
- Penny was the only member of the group to have a bra shot in a few episodes of the entire show whilst Bernadette and Amy never had any bra shots in any episodes of the entire show. Penny is the only member to have a belly button seen on-camera with her bra shots in one of the dream sequences of "The Cooper Extraction" (S7E11) whereas Bernadette and Amy never had any bra shots and belly buttons seen on-camera.
- Bernadette was the only member in the group who hadn't been seen with Sheldon's mother for long until "The Conjugal Conjecture" (S10E01), but didn't talk.
- Each member had a bachelorette party that mostly featured just the three of them.
- Bernadette and Amy were the only members in the group who hadn't been seen with Leonard's mother for long until "The Celebration Experimentation" (S9E17), but didn't talk. However, in "The Line Substitution Solution" (S9E23), they were seen with her again and for the first time she had conversations with them.
- Bernadette is the only gang member in the group who has never met Alex Jensen.
- Bernadette is the only gang member who did not spend time with Mary Cooper.
- All three members are revealed to be mothers. Bernadette gave birth to 2 kids during the series, it's been mentioned in Young Sheldon that Amy has children in the future, and Penny was pregnant during the finale.