Workgroup Manager isn't included with OS X Server. When upgrading to OS X Mountain Lion or later, the installer removes Workgroup Manager.
To administer iOS and OS X systems, use Profile Manager included with OS X Server. Profile Manager delivers configuration profiles and Mobile Device Management (MDM) for Macs running OS X Mountain Lion, Lion, and iOS devices. Profile Manager allows for configuration and management of user accounts for mail, calendars, contacts, and chat, enforcement of restrictions, PIN and password policies, configuration of system settings, and more.
A legacy version of Workgroup Manager is available from Apple Support Downloads.