So what would happen there in this situation?
First of all it would have taken a while before
anything happened. Then the cops would have removed them, gently if they didn't do any resistance. Then they probably would have gotten away with it or being given a ticket for it.
That is, of course, if it was a private or public house being empty. Being into a house with
residents in it, the residents owning the house and you refusing to leave is a pretty serious crime here.