Establish group norms.
The group always thinks it's correct and possibly moral. And has ways of policing in and out.
All that's left is to try to exert control and make sure everybody knows that you think your group's bestest.
The person most at risk in that situation isn't those in other groups (unless the groups are vastly unbalanced). The person most at risk is the individual--a group of one, with no solidarity, support, or Greek chorus saying how great and good and right the person is.
The individual could try to be a loner, but the thing about groups, esp. once they've invested emotions in their goodness and purity, is that they can't tolerate others acting like they have any right to dignity.
Kids in my school, decades back, mocked those who wore really shabby clothes. But if somebody came in decked to the 9s and wasn't already popular (thus permitted to violate the norms, at least sometimes) they were just as mocked. "You think you're better than we are?"
To some extent there is a point there--it's possible to really provoke envy through conspicuous consumption. When this one high school girl got her license and the next day at lunchtime a red Porsche showed up in front of the school wrapped in a big white and gold ribbon with the girl's nickname as the license plate number, it was really unthinking.
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