Change Auditor can only audit when group's are changed, such as when an object is added, moved, removed or renamed in a group object.
However there is capability in Microsoft PowerShell to search for unused groups as per the the following Article:
NOTE: The following article is provided for information purposes only. Quest does not support third party scripts or tools.
"Hey, Scripting Guy! How Can I Use Windows PowerShell 2.0 to Find Active Directory Domain Services Groups Not Being Used?"