The list of group members can be hidden from public view by making a group private. Only the owner and members can see the list of members, and they must bind to do so. Only the group owner can make a group private. Note that making a group private does not prevent non-members from sending e-mail to the group.
Instructions are provided in Appendix D: Private E-Mail Groups of the document, Managing Groups that You Own in the U-M Online Directory (S4277)