Well, as other people have said, the other mothers' responses to being told that their kids are involved in bullying go a long way to explaining why the kids are bullies. Even "nice" kids make mistakes and childhood is the time to correct them--not pretend that they don't exist.
If your daughter has gone to the party anyway, then it sounds like she is doing exactly what she needs to be doing--not letting these people dictate her social life. She needs to keep doing that.
I used to have similar experiences when I was a kid, but they soon ended. I now do volunteer work with kids in my free time and have often wondered why some kids end up having such a horrible time, whereas otherslike megot away relatively unscathed. My parents were the town eccentrics, I was chubby, we were poor, and my father refused to buy me new clothes because he didn't want me to be vain. All of this should have added up to sustained bullying, but didn't.
I think there were two major reasons and one was that I was an arrogant kid! People talk about things like encouraging your kid to have "self worth," but that's just another way of saying that you should make sure your kids knows she is better than these people! They are petty and immature and she isn't.
The other was that I was lucky enough to live in a small town where there were many opportunities and little competition. It was easy to get involved with things like art and dancing and sports because they were cheap and everyone could get on a team. It did me a lot of good to have hobbiesand improving at themto occupy my brain. The friends I met through those hobbies focused on the hobbies instead of on petty drama. I think that when people are bored, that's when they start creating social drama. And when we have only one group of people in our lives, whether it's family or friends or work colleagues, any drama/unpleasantness within that group takes on a lot more importance and is much more distressing.
So my advice, if you are looking for any, is to continue telling your daughter not to allow herself to be kept away from parties and other events by these girls. Keep telling her that their attitude is not normal or worth getting worried about; they are just immature, nasty little **s who haven't been taught any better and who don't have anything more interesting to do with themselves. For yourself, vent on here, but don't get emotional around her because that will rile her up and make her feel this is more of a disaster than it is. Take a calm, pragmatic approach. It may not seem like this is sinking in, but it will be. And encourage her to develop at least a couple of hobbies that will get her out of the house, away from these girls, and into contact with other kids who have interests.
You will be teaching her a very important qualityresiliencewhich will stand her in good stead throughout her life as she deals with horrible people.
Good luck!