Is it possible to set up a distribution list in the GAL such that no one can see the members of the group?
If this is Exchange 2003, log on to the Exchange server, open Active Directory Users and Computers, right-click on the relevant distribution group, select Exchange Tasks, and then select Hide Membership.
Hiding distribution group membership is actually not supported in Exchange 2007 for some reason, but you can still do it. This article provides an excellent rundown of the steps you'll need to take.
I hope this helps!