The client should be bound to a server using System Preferences.
OS X Lion clients should be updated to OS X Lion v10.7.3 or later. OS X Mountain Lion clients should be updated to v10.8.3 or later. Clients running OS X Lion v10.7 through v10.7.2, or OS X Mountain Lion v10.8 through v10.8.2, may not appear in the list when browsing.
Mac OS X v10.5 or earlier
When using the ellipsis button (...) in Workgroup Manager to browse and add computers to a computer group, clients will not appear in the list unless:
- The client is bound to a server using Directory Utility
- The client receives LDAP information via DHCP (also referred to as "option 95")
Workgroup Manager is not included with OS X Server (Mountain Lion).