Currently it's more work probably than it should be to do this, so for newer people, it's recommended they stay on large instances like lemmy.world or kbin.social to have the communities searchable.
If you're on a newer server, you can add the community you want.
Click Search (the site, not search for the community)
enter the whole url of the community you wanted - eg. https://kbin.social/m/Frenchies
Click enter and wait for a bit until it's able to find the community and you can hit subscribe button.
It's easy to do, but nothing on the UI helps you find the communities without extra knowledge, so if you're newer, it's a better experience to stay on the larger servers