Packs, flocks, alphas, generals.

There are no packs, there are no flocks, there is no leader. We do not behave in the animal sense because we do not act instinctually. What you are describing is more like a tribe than a group of friends, a group members have no choice in being part of and is fundamentally necessary to their survival normally.

The difference is in the fact that people choose to be around their friends and are quite capable of choosing to be around other people should they believe that time spent with another is worth more than time spent with their friends.

Groups are not groups, they are gatherings of individuals and should be viewed as such, if a person thinks you're more fun than each of those individuals or the collective fun that occurs when all of them gather then you will be chosen over them. If not then deal with it until such a time as you are more valuable (see: the phenomenon where mates stop going out and spend more time with their girl - viewed as more valuable).

Social circles come and go, they're easily broken by fickleness and you'll find that if you look deeper into the lives of those people they usually have other circles that they also either take part in or flit to and from regularly. Life isn't a Carling ad.