Signing in, and inviting the household

• The admin account is the Plex account that owns your server, and there is only ever one. Sign in with it the first time and it claims the install; after that it is the only account that can sign in at all. Another Plex account attempting it is refused, and deliberately not told whose install it is.
• You cannot claim somebody else's install. Once a Plex server is connected, a claim is only accepted from the account that owns that server. If Plex is configured but unreachable Kustos refuses rather than guesses, so a Plex outage cannot be used to take an install over. The plain first-come claim applies only in the gap between first start and connecting Plex, which is why an unclaimed install serves nothing but the setup page.
• Everyone else links to that account rather than having one of their own. They connect their Trakt or Plex account through a link you send them, with no admin account and without ever seeing the admin UI. Their watch history, watchlists and suggestions all hang off the admin install. Two link flows exist and the difference matters: see Link and Invite.