Aside from the no-one-turned-away-for-lack-of-funds thing so broke people
can attend, events often have a suggested donation because a film,
recording, piece of music, etc. may be licensed for non-commercial use
only. That means they would not permit the movie to be screened when an
admission fee is charged. But you could suggest donations (another
alternative would be sell food/snacks/etc.)
--mark B.