I almost always have extra guests show up, too. Like everyone else, it's usually friends, roommates, partners, or family of the guests. I've found that the guests always go to wherever the guest is, even if it's in the bathroom. They just flock to the host. So, be sure your host is where you want the party to happen. Also, my guests always bring food, even if I provide it. It's never a problem of running out. I don't serve food at a birthday party because everyone is going to eat birthday cake. The biggest problem I have with parties is everyone getting in each other's way, no matter how much space I seem to have!