Take the full URL of the community you're looking for, and paste it into the search bar of sh.itjust.works while you're logged in. https://lemmy.ml/c/yuzu (you can also search [email protected])
It might say "no results" but go back to the community page and check again. Then subscribe.
At least one user on your instance has to have searched for it before. The devs are working on making this happen more automatically, but that's how it is right now. Just one person on your entire instance has to do this one time, then it should work for everyone else there.
And note, comments and older posts won't come over immediately but you should start seeing all new posts and comments.