Those guys aren't your friend. Some of my guy friends are the worst womanizers that have walked the earth and no matter how drunk they get or how attractive the girl I am dating at the time is they would never cross that line. Even with ex's.
Those people are your friends. Are there guys who I may know who would do this type of thing, sure. But I would know better than to leave them alone with a girl I was interested in.

Honestly, it sounds like this chick has you pegged for a friend not a romantic interest. If you have been working her since summer and 5 months later you haven't closed the deal then there is something up. If you are hooking up with her this long and she hasn't asked you to commit then she is having fun and not going to be looking for a boyfriend anytime soon. If she hasn't put out by now and you have really seen her every weekend for all these months then the odds are that she isn't going to be digging your junk in the near future.

I wouldn't read too much into what those guys did. If she was at that party to "be with you" then they wouldn't have had the space to work her, even if she wasn't responsive to them. You should be far more concerned that you are pining away for someone for almost half a year and you've made no progress.