The recording sessions for Group Sex took place in July 1980 at Byrdcliffe Studios in Culver City, California. Four songs on the album – "Wasted", "Don't Care", "Behind the Door" and "Red Tape" – were originally co-written and performed by vocalist Keith Morris with his previous band Black Flag. "Wasted" had appeared on Black Flag's 1979 extended playNervous Breakdown. "Don't Care" was recorded during an early recording session for what was supposed to be Black Flag's debut studio album. After Morris left Black Flag, the tapes were shelved. Black Flag's version of "Don't Care" later appeared on the 1982 compilation albumEverything Went Black. "Behind the Door" was re-written and released as "Room 13" on Black Flag's 1981 debut studio album, Damaged. Morris and Circle Jerks drummer Lucky Lehrer collaborated on the arrangement of "Red Tape". In a nod to the latter's background as a jazz musician, the jazz principle of "trading fours" was co-opted by Lehrer in an effort to expand the narrow range of musicality that existed at the time in hardcore punk. When Morris recorded and released "I Don't Care" with the Circle Jerks, Black Flag responded by rewriting the lyrics, and then recording and releasing the song as "You Bet We've Got Something Personal Against You!" for the Jealous Again 12" EP, which was also released in 1980. "Live Fast Die Young" is a reworking with new lyrics by Morris of "Cover Band", a song written and composed by Circle Jerks guitarist Greg Hetson for his previous band, Redd Kross, who recorded it for their 1980 EP Red Cross.
Group Sex was met with generally positive reviews and ratings, and the album has been called a classic of the hardcore punk genre. Mark Deming of AllMusic rated the album 4 out of 5 stars and stated, "As such things go, it's tight, reasonably well played, the songs kinda sorta have hooks, and Keith Morris is a pretty good frontman, but if you're looking for nuance, you're pretty much out of luck. Then again, if you were looking for nuance in a Circle Jerks album, you've obviously been misinformed as to how this punk rock stuff works". Robert Christgau gave it a "B+" saying, "Like the Angry Samoans, although not as clearly or catchily, these slammers double-time metal riffs behind the rants, yielding such indelible plaints as "Deny Everything", "Paid Vacation", and "Group Sex" ".