Users with Admin access in Sitoo Backoffice will have total access to the system. Therefore, it is only Sitoo who can add additional admins to the system upon request from the subscription owner.
Add the user under "Contacts". You will need to assign an e-mail, a password and a name for the contact. Then send an e-mail to firstname.lastname@example.org detailing exactly what contact should be assigned as Admin.
Read more about the different roles här.