We went through this with my daughter (now in her 30s) while she was that age.
She was in a group of 4 or 5 other girls at school, and every so often one the "gang" was excluded from the group for some reason or other.
Then the next week the excluded one was back in, and a different girl was excluded.
It was upsetting for the excluded one at the time, but then a week later they were back in and happy again.
This is all part of growing up and learning to deal with other people and making and keeping friends.
Dont interfere, she has to learn as she may get similar problems at college or even in the work place in the future.