Our AD is not setup with the default "Administrator" (our Exchange Admins must have configured this already).Generally, the default "Adiministrator" account is copied then deactivated.
All clients use an imageSo they are in Workgroup. When you will join it to a domain, the "Domain Admins" group will add to the local "Administrators" group. So that, all admin of the domain are admin of the Workstation too.