[@Info Broker]
Depends on how active you want those groups to be, and whether or not they're represented by characters, or actual Groups that will need to be submitted and have participants before they're active?

If each gang has a representative character, you could list those gangs as resources in those character sheets the same way other players list magical powers, cool weapons and stuff. Alternatively if you're looking for these groups to be background elements to pursue and interact with, I'm sure you could just make threads for whatever spaces you want to invent on the planet to house those gangs and represent opportunities for players to interact with them.

This is all from the outside looking in though, and based on my past experiences with how to interact in PWRP. You might want to page one of the active mods just to be sure your idea goes over well. I haven't been here for years.