This happens to a lot of people, and it may happen to her throughout her entire life. I mean, adults do this too. Things I've learned:
Let it go. People who were actually *good* friends generally aren't exclusive. This means that they might get together on their own but they also include the other person as a matter of course when they are around. Example: I have a dear longtime friend who introduced me to her roommate, who I really enjoy. When my friend was in Europe, her roommate and I hung out together. When she returned, we all either do something as a group or I spend my time with my original friend. We are all adults, no one would feel excluded, per se, but I think that's just how we roll. Because we are considerate of each other, no slight is taken or given.
I have also (rather recently, and here I am, nearly 47!) been on the 'excluded' end of things. What I realized, pretty quickly, was that I was friend of convenience for a couple of women, and when they connected, I sort of got nudged out. While it stung, it led me to be open and receptive to others and I now have a much-longer, much deeper friendship from that. Interestingly enough, when I ran into one of the women, she stated that the other had also done the same thing to her: "bff' behavior one day, dumped ungraciously the next. I am glad I didn't spend a lot of time wondering what I did wrong-- that's just this other gal's MO. She burns through people and is pretty unaware about it.
It's easy for us to internalize our being pushed to the side. But the fact is, this is about *them*, those people who are happy to push people out of friendships aren't friends at all. They are lessons we learn in life about ourselves and our own boundaries. Let your daughter know that A. it's not her, it's likely them (esp drama girl); B. this is common, and even the nicest people deal with this and C. there are people out there who can be fun and inclusive. Look for those folks. Stuff happens. Don't hold onto it. Chances are, Drama Girl and the Twins may not be thick as thieves after a year or so, and it's likely your daughter will have met other kids she enjoys by then.