Honestly the group in question is of no consequence to the story other than the new policies forcing Rarity to leave, as we don't actually see them again as the story progresses.



And to be quite honest I had a whole spiel ready to give you an answer about this group, but you're comment got me thinkin', so first I'll answer your first question in a short blurb:



Originally what I had in mind for this group was basically the beginnings of just another run-in-the-mill bandit group of this AU. Bandits typically don't have elderly or children with them because they "slow them down" and are seen as a foolish burden to keep if you're just in it for your own survival. They'd have a strict "NO KIDS" rule that applies to anyone and everyone in the group, including the leader, and their overall goal is basically to live to see the end of the day. They don't think they'll live for that long so they figured they'd make it as easy on themselves as they can until they ultimately meet their doom one way or another, and with that it's basically a group of rag-tag selfish pricks who watch each other's backs simply because it's easier to have more than one set of eyes scanning the surroundings instead of all by yourself. Their idea: stay alive for as long as possible using methods that are possibly immediately satisfying: rid of the children and elders for more food, kill wandering survivors for clothes and supplies, raid settlements for weapons and the like. Very much not the most logical of thinking but it's a form of thinking some groups take up. These groups always end up dying out in the end.



NOW with all that said after you mentioned the whole dystopian thing I kinda had some inspiration so the group Rarebear was in could go like this instead for more interest:



New leadership is now in power and they want to get rid of any "useless" folks of any age. If you aren't what they consider mentally sound (which is ironic given these "leaders" certainly aren't), able to work or contribute to the group in some way, you're out, end of story. The kids can possibly stay if they prove themselves useful somehow, like perhaps farming or setting up traps or hunting. But the catch there is that, when the leadership came into power, these kids were deemed "worthy" or "useless" based on their current set of skill and not what they could possibly learn. If they didn't know something at that point then they're out, in order to make room for children who will be "pure of blood" (aka children of the new leadership and anyone they think is "worthy" enough to bear a child). They'll have strict child policies, only 1 child per pair, and only the members that's proved to be assets to the group may breed. That's the primary leader by default, but they can't just go pick a female or male and breed all willy-nilly; both parents must be high-ranking in the group to be allowed to have kids, and those who aren't considered nothing more than perhaps cannon fodder or essentially slaves have absolutely no right to have any unless a need for extra workers come up. Until then, to keep the amount of mouths to feed to a minimum, only the top of the settlement may have kids. I'm sure they'd have prisoners that they've sterilized somehow to be used specifically for sexual pleasure as well, with or without consent. They'd probably have a strange "ranking" system too to fit this overall structure, like... trying to use titles such as "king and queen" or something of that sort



As far as Rarity's concerned, she'd want to leave this sort of group not only for them threatening the safety of Sweetie Belle (which is the primary reason) but perhaps also because the new leader wanted to make her his/her Viscountess? I dunno, gotta think more on that later.



As for the leader, if we followed the dystopian format I'm gonna assume that, eventually, the high leader might put in place a "law" of sorts to make it so they don't have to worry about ever being deemed no longer beneficial to the group and, in result, be kicked out or killed. Perhaps something like... "the leader must stay in the group to teach their successor how to properly lead the group before departing" or something along those lines.