Sorry, but I dont think you've explained that very well. When you say "client" do you man user, PC or a customer?
When you say 'added to the domain', you will have already created a user account for these people on your SBS and will have also set their access level (standard user or admin user).
This means that when they join the domain and login to their PC their access rights are already set for them. It does not matter if a user was previously an administrator when they were part of a workgroup, they will become whatever your SBS tells them they are now.
In our organisation, most staff are standard users so they can do their work but cant change any settings or install files or printers etc.
Some staff have admin rights which means they can install printers and do other basic admin tasks but there are still many limitations, they are not full admin. Like they said in the Highlander Movie "there can be only one" (admin).
So, whenever something technical needs to be done to their PC I log in using the SBS admin account details. The same login details I use when I login to the actual server. This means I'm not going to get a message saying "You don’t have sufficient privileges to do that".
Actually, I think on one occasion I did join a Workgroup user who was an administrator to the domain and although I had set their account type as a standard user when I checked they had admin rights.
Hope this helps a bit.