Win 2003 – Auditing User Accounts – Who added which security group?

How to know which administrators added which security group to a certain account?
For example:
Admin A added himself to a VPN group, or asked admin B to add him into the VPN group. Is there a way to find out who added Admin A into the VPN group w/o asking him?
I read about Auditing User Accounts but it only audit who gets what security groups, not who gave the users those groups.