I've been really lucky with RSVPs, since we've only invited close friends and family. I got my first RSVP this morning with someone adding uninvited guests to their RSVP. My FMIL wanted to invite a few friends, and among them was her close friend (and her SO). I got their RSVP back today and they're bringing 2 extra people. They're older, so I know their children are grown so I have no idea who these 2 could be. We don't even really know them, let alone whoever they've added.
A phone call to FMIL to investigate is pending (she's working), but now I honestly feel like I can relate to the multitude of RSVP issues that have been posted on the board over the last month.
I think we'll probably just accommodate them, since it's not like my dad's side of the family wants to celebrate with us and we haven't had any others RSVP like this.
Link to backstory on dad's family:
http://forums.theknot.com/Sites/theknot/Pages/Main.aspx/wedding-club-boards_june-2012-weddings_why-does-my-fathers-family-have-to-make-it-clear-that-im-not-one-of-them-venting-ridic-long