That's why it helps to bring a friend or two of your own to even the numbers.
If a girl is absolutely into you she will blow off her friends to get a piece. So keep her interested, but it wouldn't help to talk it up with her friends a bit too.
You could also consider it a test. Next time it happens try something like:
"Hey! Don't take her away, we were having a good time!"
But say it in a playful manner. If you haven't already broke the ice with her friends, this is how you do it. You can easily tell how she reacts to this if she's going to be a CB or not.
But for the most part, try and roll with at least one other friend who knows what he's doing. The type of friend where you could tell him "Hey man, I think I'm going up to so and so's room" and you can peace out and he won't even care, because he already has his own chick, or he understands the rules of the game.
-Scars