I created a Drupal 9 clone of my website yesterday afternoon https://test-inv-letters.web.cern.ch/ but I am not able to add other admins to it yet. How much time does it normally take for the website to appear here https://groups-portal.web.cern.ch/ ? Or is there any other step that I missed?
When you create a clone of a website, it shares the same authorization as the parent website ( therefore admins on one website will be the same as the other).
In order to have clones that work as entirely different websites please open a ticket, otherwise just re-use the authorization procedure in the parent website and the changes will also be applied to the clone.