Hey A.! Your girls sound adorable, and seem to be developing into two very different people. I would accept that it is possible that one twin will be more "popular" than the other. Teach them now in small doeses how to cope. Teach the one how to be sensitive to her sisters feelings if she is left out, and teach the other with a smaller deeper group of friends how to cherish and value that, while being happy for her sister whose social life looks different (and not to think less of herself because her social circle may be smaller/deeper).
I think that it is really important for us as parents to use this kind of thing as an opportunity to teach our kids that sometimes one will have something and the other does not. The character quality of being happy for the other when something good happens (even if that something didn't happen to them too) is really important. And it is especially difficult when the kids are twins. I think we have a view that they will grow up super close and have all the same friends and hang out together forever. Until a man chooses one over the other...or like your situation, a friend chooses one over the other.
Especially when they are small, they should be learning in small less traumatic doses -- from you, because you love them (if they have to learn it from experiences out in the world, it might not be such a gentle learning process!) -- that they are different people, and as such may be treated differently by you when you think it is appropriate and that you may choose to give them different things or grant different privileges when you think it is appropriate. This way when it happens out in the world (because it will), they will have already experienced it in a loving environment, and know how to positively respond - both internally and externally.
Allow them to experience these things in a manner controlled by you (this situation is perfect!) and use it to teach them the right responses. Teach the one how to be sensitive to her sister, teach the other how to respond internally (not to think there is anything wrong with her, but to begin to realize that they both will have some same and some different friends.
This challenge is perfect, A.! It could be a GREAT teaching/training opportunity!