I’m using an account registered on lemmy.click, and am subscribed to magazines from kbin.social. They show up in my stream, and I can use them as if they were lemmy communities.

Conversely, I’ve seen kbin users commenting in lemmy threads.

I don’t know how much of this is the norm or the exception. I heard that kbin.social federated with lemmy, that’s when I went looking for their magazines.

But generally speaking, you don’t have to create a kbin specific account and can access kbin with a lemmy account. In practice, the scope might still be limited. I also still have a lot to learn.