Like Mastodon, Lemmy is a federated group of servers, however in order for a server (also known as an ‘instance’) to be aware of content from another instance, someone has to let that server know the community exists…

In my case, my home is on mander.xyz. If I go to the communities tab and search for Longmont, nothing comes up. From the post on reddit I knew that the home of this group is at “https://lemmy.ml/c/longmont” so I simply put that whole string in the community search, and that tells my server of its existence. Then I can immediately go back and search for “Longmont” again, and now the group shows up and allows me to subscribe to it.

And of course this works for any other communities you are interested in, but which aren’t hosted on the Lemmy server you have an account with. Once you or someone else on your server has subscribed to a community, anyone else on your server will also now immediately see that community without needing the full path.