Ah - got it now!
The Music Department choral ensembles are classes. They meet between 3 and 4 hours a week, for which you get a grade and 1 credit. The descriptions of all the groups and their rehearsal schedules are on the Music Department website under Ensembles (even though they're classes!).
If you sang in your high school's chamber chorus and can sightread somewhat, you'll definitely get in to one of the groups.