I'd say to put it right out there to those who have made noises about building a friendship. Maybe you could say "look, I don't know what I have done to offend XXXX but she seems to be making things a bit difficult when it comes to seeing you guys. I'd love to get together more, I'm not sure what to do".

That way it's out there clearly. There's no way the rest of the group haven't seen this happen before and there is a good chance at least some of them are a bit sick of it themselves. You are also not lowering yourself to her level, so if no one wants to change anything you definitely know where the loyalties lie.
And in all bytch-circles presided over by one bytch it falls down sooner or later.....not everyone wants to remain immature forever. No one ever feels safe because it could be them on the hitlist next.

hth xoxoxox