I think uninvited guest will do.  Might have some other words for it.

Let´s wait and see.

PP.

http://weddings.about.com/b/2009/02/22/what-would-you-do-about-univited-guests.htm
What Would You Do About Uninvited Guests?
Sunday February 22, 2009

Your weird cousin Bob says that he's bringing an uninvited date to your wedding. Your coworker announces that her kids are coming. And your well-meaning neighbors tell you how thrilled they are about your wedding, even though they were never on the guest list to begin with!

So what do you do? While etiquette suggests that the bride and groom call any quests who have RSVP'd for more people than intended to explain the misunderstanding, this is not an easy thing to do. Some couples don't say anything at all, but feel resentful afterward. Others take the time to write every guest's name on the response card, trying to ensure that no extra guests will be tacked on. Recently, I've seen couples trying to circumvent this problem by adding the line "We have reserved ___ seats for you" to the RSVP card. What do you think of this solution? A good one, or tacky?