Try it, when I search !functionalprint@kbin.social on the search bar, there are no results, but the community of https://kbin.social/m/functionalprint is very clearly there, and are both a part of the fediverse. What’s going on?
If no one from your instance has subscribed to the community, it can not be found normally via search and the content wont be federated to your instance.
To be the first to connect your instance to that specific community: type the real url (https://kbin.social/m/functionalprint) into the search with search setting set to “All” and you should find that community. Then just subscribe and wait for your instance to start federating with that community and you should be able to participate.
Once you have done this others in your instance should see the community listed and can instantly jump in.
So if I understand correctly it’s not enough for someone to be subscribed to any community from that instance?
Correct. A connection with just one isn’t going to dump everything from that server to yours, only the content you picked.
