Basically, how it works right now, you put the server name at the end of the community. So from a lemmy.ml account, browsing lemmy.ml, you can go to, say, https://lemmy.ml/c/socialism to go to a local community, and https://lemmy.ml/c/communism@lemmygrad.ml to go to a Lemmygrad community. Then you can subscribe, comment, vote, whatever.