I have been dealing with the same exact thing!!
My bridesmaids are driving me batty with this! One is very much single asking if she can bring someone as her date, one is married and invited another friend (someone I don't know!).
My damn photographer (works with FI) is bring a girl who is rude to me every time I stop into their work. We're paying for the photographers room and she's staying with him. She is not on my guest list for people from FI's work either
My mom invited her husbands (my, umm...2nd step father)parents - I've met them TWICE - once in passing and the last time I saw them was Christmas 2006.
My dad has asked his best buddy to go with him and I've met him once at my grandma's funeral.
My solution is that whoever is on my guests list is who is on it. I'm sending "passes" for the invited only. If the +1's want to go, fine, but they're paying for their own meals and stuff. They can join us, but I'm not paying for their dinner and stuff. I've got 9 months to go and already my list is up 6 people. At $70 (dinner, 1 drink and gratuities) per person, they are on their own.