Workgroup Manager is not included with OS X Server. If you used Workgroup Manager in a previous release of OS X, the installer removes Workgroup Manager when you upgrade.

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 such as iPad, iPhone, and iPod touch. 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 for use on Mountain Lion.