I was bullied in school and had relatively few friends. Mom put me in Girl Scouts, and after I got to Juniors (green uniform) and went to 3 meetings, I flat-out refused to go anymore because I didn't like the behavior of the girls. They were mean to the leaders, even.
So Mom made me join 4-H. In retrospect I see that she was desperate to provide me a "safe" place to be involved and to interact with other kids. Somewhere that would be free from the dynamics of school.
So I'm a bit torn. But I also have seen, from being the picked-on kid, how an ADULT'S actions can so competely make a difference in the behavior of the bullies.
(and, in fact, the 4-H group I joined had as a member a couple of girls who were mean to me at school. They behaved differently in the group, which was really an eye-opening lesson to me. *I* learned to view them as three-dimensional people, which helped me a lot with the "being picked on" part.
So I think you need to be prepared to be really proactive. Speak up in the moment with chastisement (but be sure you're seeing clearly and aren't reacting to reports of past actions). If necessary pull her aside and talk to her. You'll have specifics, so you can bring them to her mom.
And you can also, if you're paying attention, figure out how to teach the other girls how to deal with her.
And I'd also make this the year that we really focus on the Girl Scout Law. That's a tremendous tool. And you can use it if it becomes necessary to ask her to leave (not sure what the rules say about that).