The way Lemmy works right now is you search a community in your instance for it to then get shown, so you need to first discover that community from elsewhere. With that in mind, what are some growing communities that you discovered that could get some more love?
You only need one username for each group of federated servers/instances. Click Communities at the top, then click All under List of communities. that should list all communities known to lemmy.ml. If a community is not known, you would need to search for it (eg. [email protected]).