"I'm so sorry, AG, but we've already got our prom group established, and our plans are set. But it sounds like you and AG1 and AG2 have a great start on a group, you guys should start making plans--you can do prom your own way that way!"
It's hard to be in either position. There's kindness and cuts no matter what they do, but DD's feelings and experience are important, too. It'll be a good experience in politely and subtly--but firmly--standing her ground, and for AG, it'll be an experience in making her own way, socially.
I believe AG1 and AG2 are actually part of Joraemi's DD's group, and AG actually has no one to go with.
Actually, I just rechecked the OP, and it says that AG1 and AG2 aren't part of the DD's prom plans group, either. So it seems reasonable for the DD to suggest that AG form her own group with the other two.
I really feel for the OP's DD...actually, I feel for both DD and AG. Having been one of the "rejects" in high school (although I was, fortunately for me, one of those kids who functioned pretty well on my own), I know it hurts to be told that someone you consider a friend doesn't want to be friends with you, or at least not close friends.
And yet, on the other hand, I've been on the DD's side of this situation a few times as an adult. And it's not easy to tell someone to back off a bit, either, when there's nothing really "wrong" with them as a person except that they're bit clingier than I'm comfortable with, or even just someone I don't like as much as they seem to like me. I know there's nothing wrong with enforcing my boundaries and making decisions about who is my friend and who is an acquaintance, but it still feels a bit mean to have to tell someone that I really don't want to pursue a friendship further.
So I think this is an excellent time for the DD to learn that it's ok to have boundaries and to make those decisions, and how to do so politely, effectively, and as kindly as possible. And in this case, it may not be as possible to be as kind as the DD would like to be, because I don't think AG is going to "get it" unless DD is quite blunt and firm. In the end, I don't know if it matters whether AG has a crush on DD, or whether she is just socially clueless and trying, ineffectively, to be friends, because I think the DD's actions need to be essentially the same, regardless of what is "driving" AG's behavior.
OP, I think your DD needs to tell this girl (preferably privately, and very soon), "AG, you seem to have gotten the impression that you'll be going to prom with the group I've decided to join. I need to tell you that it won't be possible. We've already made plans, and it's not possible to include anyone else. Why don't you see if AG1 and AG2 have plans yet?" And then if AG tries to cling to DD at the prom, DD should take her aside and tell her, "AG, I am spending time with X, Y, and Z tonight. You need to go spend time with other people now." Or something similar.
DD should be prepared that she may need to get more blunt than that, possibly escalating on up to "AG, I do not want to spend time with you. Leave me alone." Hopefully it won't have to go that far.