I just spun a container up to test this, it disables the registration and only allows invites after the initial account has been created, it returns an error if someone accesses the registration page and tries to create an account, www.focalboard.com/guide/server-setup/#registerin…

There doesn’t appear to be anyway to manage users or non-owned boards, you can reset a user’s password through commands, www.focalboard.com/guide/admin/#resetting-passwor…