There are terms for this. The communities are called communities and not Sublemmies. And Lemmings is a very specific term which excludes all the other services in the Fediverse which can interact with Lemmy communities. I prefer an inclusive term like Fedinauts or Fedizens.

I’m not a fan of sumlemmies. Since “communities” is a mouthful, I propose “communes” or just “commies”.

I really hope lemmings doesn’t become the norm