How to specify group id for a user-defined group
You can still use Workgroup Manager on Mountain Lion.  You just have to download it and install it.
You can also do it using command line tools, but since you're used to Workgroup Manager, just use that.
Added May 2018:
In later versions of OS X, you can edit the group IDs directly from the System Preferences → Users &amp; Groups control panel. Ctrl-click the group name and select &quot;Advanced Options...&quot; from the popup menu. That will display a form that lets you change the group ID (as well as the group name and the group UUID). Be careful though, as this just lets you change the mapping of number to name. Any resource associated with group ID 1025, for example, will still belong to group 1025, even if you change the group name associated with that ID. Which resources are associated with a group by ID, which by name, and which by UUID, I do not know.