The chorus at my daughter's high school (I also graduated from there many, many years ago!) does a grand performance of Messiah (Part the First, plus the Hallelujah Chorus) the evening before Christmas vacation starts.
Parents and alumni (at least, those who can read music and sing) are invited to grab a score and rehearse (and perform) with the kids. It is a great deal of fun. Cast of hundreds, plus an orchestra and a harpsichord (well, really, an electronic keyboard set on 'harpsichord'). The singers from the small touring chorus plus an occasional ringer sing the solos.
We always have a very good time.
The university near us did the Messiah that way. We brought our own music and I got to sing the solo part for “How Beautiful Are the Feet.” Nobody else apparently knew it.