1. you should try making friends with her or his best friends.
2. tell your BFF how you feel about her new friends.
3. Try making other new friends if this makes you happier.
If you pick number one, try planning a sleepover or something so you can start bonding.
if you pick number two, try explain how you feel, but remember these are your BFF's friends if she wants to be friends with them, You should be the BIGGER person it let it happen.