Although they declined the invitation to perform because they had sworn off touring permanently, all four of The Beatles were on the festival's board of directors. George Harrison had helped recommend Ravi Shankar, and Paul McCartney had pushed for the organizers to sign Jimi Hendrix, who was unknown in the United States at the time.
The performance of Janis Joplin and Big Brother and the Holding Company was actually their second time on stage that weekend. When they performed the day before, their manager didn't want their performance to be filmed because he didn't want his charges to be exploited. However, when their performance was such an overwhelming success, Janis and the band decided to go behind their manager's back and performed the next night for the purpose of having their performance in the film.
According to John Phillips, Janis Joplin was a nervous wreck before her first performance at the festival and was drinking Southern Comfort and literally shaking with fear backstage. When she came off stage, she was crying because she couldn't believe she had gone over so well.
It was revealed several years later that Cream was going to be invited to perform at the festival, but their manager refused and didn't tell the band for several years. Eric Clapton became furious when he found out.
This was the next to last performance for The Mamas and the Papas. They performed one more time before breaking up a few months later.