In the table in WC user schema that manages server side workspaces (epmworkspace), the workspace row in the table stores the user who creates the workspace and an attribute that qualifies the server side workspaces "Viewexplicilyset". I believe in Creo, your CAD CONTRIUTOR" user will not be able to use this WC created by the Admin.
If you want them to share a server side workspace, test to see if they can use the server side workspaces PDMLink automatically creates and associates with the Product Container.
Hope this helps... 