Thanks for your question. Unfortunately, your daughter's friends' responses to each other sound all too typical of young adolescents. Little cliques and subgroups are springing up and dissolving almost continuously. The important thing to communicate to your daughter, I think, is that, although her friends' behaviors are terribly hurtful from time to time, they are also normal for this age, and everyone will improve as they get a little older. In the meantime, your daughter has to bear the major responsibility for how she will act, what values she will choose to guide her behavior, how she will survive the law of the jungle these kids live by, and so forth.
Many kids feel that junior high and high school are a kind of hell they just have to get through.
Your responses to her as you have reported them do not sound at all contra-therapeutic. I think you need to express confidence in her ability to deal with her friends and to express what she means, eventually. It sounds a bit from your letter like you may be just a little too involved with your daughter's pain for her own good. Your reminding her that kids this age can be pretty nuts and pretty cruel seems helpful. When they do, they are not being very good friends.