meet her friends. =D She's introducing you to her circle maybe? BUT....if it was me, I'd probably start flirting with the hottest, non-slut girl I can find.
Try meeting ppl at the party and start up a convo, you can also talk to other girls, your gf will come back to you ^__^
Man, you dont know how lucky you are! Going to a party where your GF wanders off, so you can get chatting to girls. When your GF starts complaining abou it, you have the perfect counter and blame her for leaving you behind like that. Sounds like a licence to chirps to me.
i agree with rain. once your gf smells you with another grl, she'll be back by your side as fast as counting 123. or you can party like a rock star =DDD