You can add existing admins to a site through the account management page. Simply follow these steps:
From the Company Dashboard, navigate to Manage > Users
Search for the admin, using their name or email. (If they don't already have an account, you can click the "add user" button)
Click the three dots next to their name, and select Add to Site As Admin
Next to the desired site, click Assign
When the admin next signs in, they will see that site from the site selector page.
Can I remove them from a site?
Yes, simply follow the above steps and click Unassign during step 4