Lou Adler was the producer for both Barry McGuire and the Mamas and Papas. McGuire was the one who introduced them to Adler, and in return they offered him the chance to do “California Dreamin’” first. All the sources I find credit “the Mamas and Papas” with backing vocals, not just the girls. Moreover, the story goes that the harmonies were so great that the group decided to put out their own version, which is another indication that all four sang on it. Barry’s voice was wiped out (mostly: you can supposedly hear whispers of it in the background of one channel) and Denny’s vocal laid in, along with the terrific flute solo.
“California Dreamin’” is one of the greatest rock songs of the era, which means that nothing else they sang could live up to it. All of us should have such problems.
My next favorite of their songs is a b-side I found on a boxed set, “Once Was a Time I Thought.” I don’t know if the original did this, but on the set it starts with the group struggling to get their harmonies together. Then they do, and the result is possibly the most beautiful vocalizing of the sixties. One minute of utter perfection.
That’s the sad story of the group. They had minutes of utter perfection, and destroyed themselves the other 59 minutes of the hour.