Usually you don't need accounts on other instances to interact with their communities, e. g. my account is on lemmy.world, but I'm subscribed to 3 GW2 communities on 3 different instances one of which even being a different platform (kbin and not lemmy).

These communities now appear as if they were on my instance, but are in fact local copies which are synchronized/federated using the ActivityPub protocol.

Note that these communities are not merged, i .e. they still appear as separate communities, but there is a (pretty popular) feature request for this on GitHub.

Support for grouping communities / multi-communities #818

I don't know whether federation from lemmy to kbin is currently supported. I'm only using the other direction.