Use the number of her friends to your advantage, and make the number of friends you have grow. I know I have a pretty big collection of friends I'm not really close to (and some who are extremely close and important to me), and that's because when I meet someone new and they have cool friends, those friends become my friends.

I've also managed to worm my way into the very heart of one group in the space of just a few months, just because I'm the sort of person they get along with. Don't feel like you aren't important just because she has a lot of friends. You're the only one who's actually dating her and that makes you special.