Group Managers cannot make changes to group

I have some groups which I would like the Group Managers to modify group membership.

I've followed the following procedure to allow them to modify the group membership but not create new or delete groups:

http://iconraja.wordpress.com/2011/02/25/allowing-end-users-to-manage-distribution-group-membership-in-exchange-2010/

This procedure essentially you create a new management role based on "My Distribution Groups", but you take away the ability to create or delete groups.

But even after doing so, Group Managers cannot modify group membership.  What am I missing here?

They get the following:  Changes to Public Group Membership Cannot be Saved.  You do not have sufficient permission...

Bobby Pendino