Gavrillo said:
...Can someone tell me if it's okay to go along with your American exchange students, who are apart of a wider cultural-exchange group, to a messed-up party and make out with 10+ Americans?
...If so, what do I tell me girlfriend?
Is it going to make a difference what people on here suggest you to do? If you couldn't prevent yourself from doing something which (presumably) you feel guilty about afterward, then do you have the willpower to tell your girl the full story? Imagine your girl went and did the same thing with 10 american guys -- goths, at that (because obviously it's a contributing factor, right?). How would you feel? What would you want to happen? No doubt you'd want to know...
Admittedly, in my opinion, it is a little better that it wasn't with just one person: with such a large group it seems to indicate that the emotions you felt about the girls would have to be less intense, and that you were simply acting on lust... but whether your girl sees it that way or not, I don't know.
Kick yourself, tell her, hurt for it, bear the repercussions like you should, rather than hiding from it. You'll learn, and not do it again.