Yeah, overnight guests during Welcome Week is a no-no - even other Mac students aren't allowed to stay over in different buildings. As for inviting friends to events - it's not common. Honestly, I've never known anyone who's done it, with the exception of non-Mac students coming the outdoor concert (which anyone can technically see if they come onto campus). For most other events, it's usually very residence-oriented, and since the idea is basically to get to know the people you're living with, any "outsider" probably wouldn't blend in very well. Also, since many events need a MacPass for admission, there only are a limited number which they theoretically would be able to attend, anyway.
But really, you shouldn't want to be doing that anyway! A big part of WW is meeting new people, not sticking to the same old group of friends you had in high school. You (or your friend) would be missing out on a great experience if they left their WW to go attend someone else's. Really, it's a once-in-a-lifetime opportunity, so I'd take advantage of every minute of it