“All” picks up any communities your instance knows about, whether you personally are subscribed or not. My understanding is that once one person from your instance subscribes to something it starts federating, but unless someone does that it stays separate.

While I agree being able to search for communities needs a little help if it’s a brand new community outside of your home instance to make it less confusing, it makes sense from the standpoint of having to tell the community to talk to an external instance by having someone from the external instance subscribe.

Have them go to search, make sure everything is searching all, and have them put https://kbin.social/m/LucidDreaming as the search query.

There’s a solid chance they won’t be able to see anything posted prior to when they subscribe, but at least that will make the community federate with that person’s Lemmy instance.

That is so exciting, all of us playing together in here is such a cool concept.