Okay, well I know that sometimes girls have friendship problems, I know I have, so I wanted to help you because I know what it feels like.
Well, say you are in a group of 3 girls. So there is Amy, Sophie, and Lilly(these girls are fake). Say Sophie and Lilly start becoming best friends, and are starting to disinclude Amy. Amy feels really sad, and really wants to be in the group with them.
This happened to me once, and I was the one being left out. It is not fun at all. I didn't do this, but what you should do is sit down with them and say "you know, I am kind of feeling left out, and I was wondering if you could start to include me a little bit more." They might actutally listen to you. Me and my friends are all best friends now! So, I hope none of you feel this way, but if you do, I hope this helped.