All have lent their voices to the soundtrack for a documentary about autism, which will be released Tuesday on iTunes.
"Wretches & Jabberers" follows two men with autism as they travel the world, aiming to change attitudes about their condition.
Musician J.Ralph said he signed on to compose the score for the film, but was so moved by the men's story that he wrote another 20 songs - a complete soundtrack - and asked some of music's best-known artists to give voice to them.
Stills said using the men's own words "as the foundation of the lyric was what made our song fly."
"I am extremely proud of our work," he said, calling the effort a "truly remarkable experience."
The documentary, directed by Oscar winner Gerardine Wurzburg, will be released in theaters in April in honor of National Autism Awareness Month.