Although in this case I'm the Grandparent, my Grandaughter stays with me from a Friday until Monday, she is friends with 3 other girls, one of these girls tries to exclude my Grandaughter from the group, she tries but doesn't usually succeed because the other two girls want her to be involved, it must be a bit awkward for my Grandaughter knowing this one particular girl doesn't always want her there, and 'tries' to keep her away from the other two, how can I advise her without getting too involved, when my Grandaughter speaks to me about it.
Why not ask the third girl over without the other two, see if you can pick up what is going on. Usually it is a group of three that has a problem leaving one girl out, not a group of 4