One of the eventual goals of kbin is to be compatible with the rest of the fediverse and to "just work" with the rest of it. By way of comparison, Lemmy is basically just a federated Reddit, so, this is actually one of the reasons I chose to go with kbin.

From a practical standpoint, it'll likely be a while before that becomes a reality, but I like that as a goal.

I'd say see if you can search for the instance in the search (once we're 100% federated) and if it doesn't work and it's something you want to participate in, unfortunately, you'll need to go there directly.

Also - Your kbin.social account can only log into kbin.social, you can't use it to log into any other sites (this would include other kbin sites, lemmy sites, mastodon sites, etc...), what you can do is see content from those sites by subscribing to them or on the front page (non-subscribed)