XADM: How to Find Distribution Groups with Hidden Membership
To find distribution groups that have hidden membership, use the Active Directory Users and Computers snap-in to search for objects that have the hideDLMembership attribute set to TRUE:
1. Start the Active Directory Users and Computers snap-in.
2. Right-click the domain, and then click Find.
3. In the Find box, click Custom Search.
4. Click the Advanced tab.
5. In the Enter LDAP query box, type (hideDLMembership=TRUE).
6. Click Find Now.
All of the objects that are displayed have the hideDLMembership attribute set to TRUE.
More: http://support.microsoft.com/default.aspx?scid=kb;en-us;288342


Can't View Hidden Membership in DG After You Update the Recipient Update Service
If you are a member of the Domains Admins group, but you are not a member of Account Operators and Exchange Domain Servers groups, you cannot read the membership of groups that have hidden membership after you update the Recipient Update Service.
This problem occurs because the Recipient Update Service removes the read property permission for the Domain Admins group.
To work around this problem, you must be added to the Account Operators group. To do so, follow these steps:
1. Click Start , point to Programs, point to Administrative Tools, and then click Active Directory Users and Computers.
2. Expand Domain, and then click Built-in.
3. In the right pane, right-click the Account Operators group, and then click Properties.
4. Click the Members tab, and then click Add.
5. Click the user who you want to add to the Account Operators group, and then click Add.
6. Click OK.
7. Log off from the server, and log on to the server.
More: http://support.microsoft.com/default.aspx?scid=kb;en-us;811909