In accordance with the principles of the open source movement, membership in the Assembly would be granted to any Wikimedian who requests it.
Then why have it as a distinct group? You're talking about "The Wikimedia Community", so call it "The Wikimedia Community", there's no need for an assembly. Once you realise that, it also becomes clear that these "subgroups" are, in fact, just the naturally forming groups of people interested in a particular policy (or whatever) that discuss it on that policy page.
You've just invented meta. It's a good idea, but it's so good it's already been done.