I just joined. My understanding is that you pick your home server (I chose sh.itjust.works) and then you can add the local communities from there, as well as other communities on other servers. SJW (unfortunate acronym, IMO) had a list of local and remote communities that it already knew about that I could easily choose from, and my understanding is that there's a way to add more, but I didn't look into that because it had all the communities that I had considered joining directly anyhow.

It was pretty painless, though I haven't tried actually posting anything yet.