Harsh. If you leave a friendship group because you were being left out or ganged up on, it's probably just how that friendship group operates. Observe them from a distance a few weeks later and they will be doing the same thing to somebody else. It's only the left-out person's fault if they hang around vying for the queen bee's favour then participate enthusiastically in ganging up on the next victim.
Clearly, not all friendship groups are like this. But in secondary school this dynamic was more common than not, school SAHMs often behaved similarly, and my last workplace was unfortunately a prime example.
You can't really point the finger at someone who prefers to keep their distance, as if they were so toxic no friend group would ever have them. In reality it's the toxic people and their spineless hangers-on who are the ones pushing others out: the fun's in the bullying, and if there's no obvious newcomer/misfit around they will just turn on one of their own.
For me, SATC style female friendship is like Disney style true love: it probably happens for some people, but the odds are low because too many people are shallow, competitive over ridiculous things and fundamentally transactional. So you get on with your life as best you can, and if you happen across like-minded people, so much the better. It helps not to take rejections too personally and not to put off doing things that you've always wanted to do, but would feel like a saddo doing by yourself. Life is short - but it can seem painfully long when you're in the wrong company!