In 1, the answer could be any film, whereas in 2, you expect an answer that selects one film from a restricted set - for example, you know what was showing in the cinema last night, and you're asking which of those films they went to see. You could also ask, 'what film did you watch?' if you don't know what their options were and expect any answer.
The number of people you're addressing is irrelevant here. The situation in which you're asking a question 'about a specific group of people' would be something like: 'which teacher takes your class?' You're asking the question about teachers (and you know all the teachers in the school, so you expect the answer to be one of them).
Note, though, that in the real world this what/which distinction is very often relaxed. You might especially come across 'what' used where 'which' might be more appropriate. It's not a hard and fast rule in casual speech.