There is no way for an instance like lemmy.ml to know about all lemmy servers, because there’s no central server keeping track.
-- https://lemmy.ml/comment/447791
I was reading the above comment and it made me think about the relays and how they could help lemmy/kbin/other Group software. Relays were a solution the microblogging sector of the fediverse used to jumpstart instances, which would have an empty federated feed making it difficult for local users to find new users to follow.
The way I envision it working would be a community relay would be an AP server with a Service or Application type Actor. This actor could receive a copy of a community's Create message and could then Announce that community to its followers. Lemmy/kbin instances could follow relays to be alerted of new remote communities and notify the relay about their own new communities.
In this way, users/instances could learn about new communities across the fediverse in a participatory way without relying on a central authority.