Collection of people assembled for a particular purpose, (to watch a movie) may be called :

Audience

An audience is a collection of people who have assembled for a special purpose, may be to watch a cricket match or a movie. Audiences are generally passive but sometimes they go into a frenzy and become mobs. In mobs, there is a definite sense of purpose. There is polarisation in attention, and actions of persons are in a common direction. Mob behaviour is characterised by homogeneity of thought and behaviour as well as impulsivity.