You must be logged into Windows and DeltaV as an administrator in order to add users. In a domain environment, you must be a domain administrator to create domain-level accounts.
DeltaV accounts are global. Windows accounts are specific to each workstation. A user's DeltaV account is only usable on a workstation when the Windows account is also enabled for that user on the workstation. You must enable a Windows account for each DeltaV user on every workstation on which that user runs the DeltaV system.