I know the feeling! When everyone gets along it's really good, but after a while two people stop talking, and then it becomes difficult for everyone else. Or someone gets real bitchy for some reason and are 'left out' of the group.
Unfortunately, I've learned that there is no way to go back to the way things used to be. Human beings are complicated and relationships between two people can be difficult -- imaging trying to keep a relationship with a whole bunch of people!
The only advice I can give you comes from an episode of The City: "Stay refined, stay elegant and stay the sweet person that you are as much as possible... Keep really good, strong people around you. The other one's will just fall to the wayside." (Rachel Roy).
Don't waste your time on silly, immature people or situations.
Someday, these "groups" you speak of will no longer worry you. You will find that in adult life, there is not such a division among people, and that harpy friends, and cousins, will no longer impress you nor make you feel like an outsider. Being a teen is hard. When you grow up, you'll look back on this and laugh. At them.