Hey
Well, I've invited college friends over and it seems to depend on the actual sim. If you ask them to "sleep over" or "stay over" they'll stay untill you ask them to go. I think the only problem is when all your playable sims leave the house, then they'll go back because no one is there.
I know that I invited a whole bunch of college friends for my sim's wedding but they arrived the morning of the actual wedding so I can't really comment. I do remember them staying for a few days afterwards though.
