While sit can refer to the act of moving to a sitting position, it is far more often used as a stative verb meaning "be in a sitting position". If we want to refer to the movement, we generally say sit down.
So, the perfect have sat nearly always refers to an extended period, extending up to the present - often with a time:
We have sat here for two hours now.
I think in your case what you want is
A lot of people have sat down, waiting for the movie to start.
That is possible, but focuses on their act of sitting, so I would say that
A lot of people are sitting waiting for the movie to start.
I have omitted the comma in the second case, because it feels more natural without it. "Sitting waiting" is quite a common expression, though I'm not sure how it's best analysed.
(In Yorkshire, where I live, many people would say A lot of people are sat waiting for the film to start, but that is not standard).