ActivityPub federation works on an opt-out basis rather than opt-in.
So kbin.social federates with EVERY other server that also speaks ActivityPub, unless they're either blocked by kbin.social, or the admins of the other server have decided to block kbin.social.
It doesn't matter if it's other kbin servers, Mastodon, Pleroma, Lemmy, Calckey or whatever - as long as they talk ActivityPub then they can federate with each other.