I'm not sure what you mean by "signing up to kbin"?
Your account on the Fediverse only works with the server you signed up on, but your server lets you communicate with accounts and groups on other servers.
It's like with email: you can't use a Gmail account to sign in on Yahoo Mail, but you can still send emails between Gmail and Yahoo Mail accounts.
I don't think Kbin has Google or FB login by the way?