OnPostModify - Group Membership Changes?

Hello,

Based on a group membership change which would be made by an administrator through the ARS MMC or Web Admin Console, I want to pickup that change and run a script that will change some other attributes.

I've got the script working fine - but the script doesn't seem to kick off when I make a group membership change.  If I modify some other attribute - location, telephone, etc. the script goes.

Is this expected behavior or am I missing something?

Thanks in advance,
Joel