To support decentralization and spread, should lemmy.world close registration at some point to prevent a performance overload due to too many users? Of course, if registration is disabled, there could be a hint placed somewhere near that from other instances you can interact with content on lemmy.world just like you had registered on it. There could be a link to join-lemmys instance overview.
They do have access to every community. But it won’t show up in the community tab until someone in their instance have already joined that community. Correct me if am wrong.