Hi all, wandering around random small Lemmy servers seeing if I can be of use. To pull communities into your instance, you need to first have someone subscribe. Useful picture in this comment, but basically you go to "Communities" at the top, and start a search with the full URL of the community you want (https://lemmy.ml/c/lemmy, for example) and change "Communities" to "All" in the search options.
Once you've clicked through to the community and subscribed, it'll be pulled in forever. If you want to dig around for communities to subscribe to, go to the community finder.