Use the Add contact dialog box to specify users or groups as participants in workflow processes, or to assign permissions for workflows. You can select domain users and groups; you can also use fields to designate users or groups based on role-based field mappings. You can also select Active Directory users via Userid and User GUID if you have imported Active Directory data by creating an A.D. Synchronization Package and imported records into the User Profile form. See "Synchronizing Active Directory data" in the Synchronization wizard help.
If the workflow includes approvers or task recipients from different domains, a trust relationship must exist between the domains so that the users from both domains can log into the Web console and complete their assigned tasks.
For more information, see Adding workflow participants.