L.R.
Darn, I had a response going and it got erased.
I was the one who suggested the "Queen Bees" book. It sounds like this girl is a classic queen bee who is now signaling to their entire group that your daughter is on the outs socially in her eyes.
It is typical that your child is defending her, despite the fact your daughter is realizing this girl's a jerk. Kids are very loyal in the face of parental criticism of another kid, period. Be sure to be very neutral and nonjudgmental in your discussions of this girl -- just point facts out, don't add judgements. Your daughter is getting there on her own.
I would not tell her about the party. A couple of reasons: Your daughter will find out from peers. That will hurt, but let her navigate it with the groundwork you have laid about this other girl's behavior. Another reason not to tell her yourself: She will know you have inside information about her group's doings. That will disturb her and could make her clam up more around you, which is exactly what you do not want. Also: Unless she is seeing girls in this group over the summer, she may not hear about the party for a while, maybe until school starts, and then she may not care nearly as much because she may have realized this girl and her pals are not worth caring about. If she asks you if you knew, I would say (and yes, I know it's a lie): "I knew X had a birthday coming up and was surprised when there was no invitation in the mail or by e-mail, but when we did not hear anything, I figured there was nothing to hear about."
I remember about the small GT class. That is a huge pity. Why do your daughter and this girl "typically pair up for projects"? Your daughter needs to have the strength this fall to arrange another partner before a project and then say, "Sorry, I'm already working with Sally on that project" and stick to it, when queen bee girl insists your daughter be her partner. She really needs to nip the academic partnering in the bud. Also, does the class rotate as different groups to different subjects? (Our GT class does.) If so, talk to the teacher and ask that your child be put into different groups for anything for which they are put into assigned groups. It would be good for your child not to be in ALL the academic subjects AND every "special" like gym, music or art with this girl. I'd also encourage your daughter to do things at school that you know don't involve this girl -- audition for the school play or be in the chorus or take band or whatever, but where this girl isn't around.
I remember that you are the GS leader. Is the whole troop made up of girls from this one school, even this one class? That often creates problems of way too much togetherness (I'm a GS leader too). I would seriously consider telling your coordinator or SU person or whoever helps put troops together that you need new blood from outside ASAP. Surely girls are seeking troops. If things really are as insular as the whole troop being from the same class, -- I don't know what to say. I would not want my daughter to stay in our troop if she were in this situation and I would find it difficult to be a neutral leader if a child like queen bee were in our troop!
Be sure your daughter gets some outside activities that involve kids who have nothing to do with her school. My daughter dances and the kids at her studio aren't from her school so it's a world apart from any social issues at school. At the studio she can be known solely as herself, and as a dancer, not as someone's BFF or someone's project partner or the girl who went to camp with X, etc. Does your daughter have an interest, sport, activity (other than GS) where she is able to have fun, shine, and not have any school-related baggage? That could help her get through the school year next year.