The above explains the process of 'Directory Trusting'
To use Enterprise IDs or Federated IDs, start by creating a directory to which you can link one or more domains.
Sign in to the Admin Console and navigate to Settings > Identity. Navigate to the Directories tab, click Create Directory. In the Create a Directory screen, enter a name for the directory
The ownership of a domain can only be claimed by a single organization.
Directory trusting allows a directory owner to trust other system admins (trustees). After this, trustee organizations in the Admin Console can add users to any domain within the trusted directory.
To summarize. If you plan to use Enterprise or Federated ID on your Admin Console, you must add the domain associated with your organization. If this domain was previously added by another organization, you have to request access to the directory containing that domain as a trustee.