@julian There's no "main" vs. "sub", other than by default all users join (and can leave) the "Big 6" official Ravelry forums.
Groups are all subject to Ravelry's TOS but can be created by anyone, any topic. There are lots of abandoned or inactive ones, and they could probably do a clean up (but also probably ... don't need to bother?)
Ravelry users belong to as many groups as they want to bother with, under one ID.
It's a unique site. Even if there was no emulating it, it's a case study.