I never assume that my dogs are invited. Never mind the fact that they are probably much happier staying home lying on the lounge in my son's bedroom watching him play xbox (although there's probably more sleeping going on than watching).
I do have relatives (in-laws) who assumed that it was OK to bring their dog to our place for Christmas. We got a heads up in a phone call a few days before Christmas, and my husband rang back and asked them not to bring her (there were circumstances that I won't go into regarding another dog we were looking after). Unfortunately, the answer was "we'll just tie her up out the front, it'll be OK." Grrr...
Anyway, as if I'm going to see a dog tied up out the front of my house for three days. I introduced her to our two dogs and it all went swimmingly. I'm not sure what I would have done if it hadn't worked out, but it was quite stressful thinking about it beforehand. As it was, I had to give our dogs time out from their children, our old boy kept leaving the room every time the 1yr old entered a room. Smart boy, I think he knew she was a bit feral. Admittedly, our children also needed time out from their children too