For the last 25 years, hundreds of singers have gathered in Stanford Memorial Church, to sing and play Handel's Messiah. Instrument players just show up and search for their music. Choral voices gather and belt out both their own voice parts, and any others they've always wanted to sing from basso profound to the highest soprano. Everybody sings the solos.
Check the schedule at Stanford each year. The Sing It Play It Yourself Messiah is always on the evening of the last day of fall finals.