I'd trap them with a Hav-a-Heart trap outside the attic, free them somewhere far, far away, figure out how they're getting in and board it up. They come out at night, but home repair isn't something most people like to do when it's dark, especially if you have to work on the outside of the house. It'll be easier to find the holes, though.
Then again, if someone in your family were to go up there and have a stern talk with them, they might find somewhere else that isn't infested with humans.