You sign up on (or create) an instance that defederates from them.

I’ve found that similar communities on different instances can have VERY different experiences in terms of the community and attitude. I know I’m generalizing, but I feel a lot of Lemmy.world instances are more negative and hostile than similar instances on beehaw.org. Personally, I’d rather they stay separate. That way I can be more precise on dialing in the experience I want.