If for example you would want to subscribe the liminal space community on lemmy.ml, you could search for the link https://lemmy.ml/c/liminalspace on beehaw.org.

This will make your server look up that url and figure out that there’s a lemmy server as well with a community that you can subscribe to and in the future you will find posts in your home feed.

You can also just browse lemmy.ml and search for a post url on beehaw, (for example this: https://lemmy.ml/post/165246) and click and subscribe on the community there.

Only one person has to browse a foreign server and subscribe to a sub before others can just see the sub on the communities (https://beehaw.org/communities/listing_type/All/page/1) tab and the posts of the sub in the All tab on the main screen. You’ll notice that the communities site already lists the liminal space sub because I searched for its URL on beehaw.