I usually check Amazon and find a book to read with the child to help with such a situation. I’m not sure what her issues are… is she being bullied? Is she a loner? Does she lack ways to communicate? Is she not as street smart as her friends? Your might just need to speak to the teacher and get her to change her group of friends. You might just have to invite one child at a time to the house and coach her beforehand on games to play.
(I personally hate having to deal with a group… I prefer speaking 1-on-1 with people.)
If the issue is severe or if you don’t know how to deal with it, def follow snowbunny’s advice.