Funextra:
Oh! A college frat? I wonder what the initiations would be like!
Bellyempire:
A squashing session by all the girls in the bbw sorority,
Be at the beck and call for all the girls in the bbw sorority
Personally, I'd go with a secret society. Mind you, it's fiction and you can do what you want. But, to me, a kink based fraternity makes no sense. Plus, it makes your world building constrained.
Meanwhile, a secret sociaty of FAs and feeders has no such limits. You can do whatever you like and make it as fluffy or as dark as you want.
A free idea to anyone who wants it:
Character A is dating Character B.
Character B is part of the society and is conflicted about fattening up Character A. They are in love with each other, but Character A is a normie.
Fluffy Route: Character A discovers the secret society. Because they love Character B so much they put on weight for them.
Dark Route: Members of the society persuade Character B to secretly fatten up Character B. Shenanegans ensue.