When you add a project group, the audit log only shows the numeric id.
2010-05-28 17:51:57 - admin - PROJECT Project Group id=31 - Added Project Group
It should include the details as well (at least the name and (text) id as entered on the web page). Similar to the logging when a Build Definition is added.
2010-06-01 21:33:49 - admin - BUILD_DEFINITION Project Group id=10:clean deploy --batch-mode --no
n-recursive - Added Build Definition