This has nothing to do with C#, and everything to do with Active Directory for Windows Server 2003/2008. If you go to 'Server Manager', go to Roles->Active Directory Domain Services->your.domain.name->Users you will see a list of users and groups, and when you double click a user and go to the MemberOf tab you will see all the User Groups that user belongs to :). Note that there's some built-in user groups under BuiltIn (rather than under Users) that you won't see in the users list.
Try posting it in a different forum for better/faster results next time.
Security tab contains groups which gives access to SHARED MAILBOXES (created as USER ACCOUNT with email adress)
Basicaly - we have about 300 mailboxes, and access groups to those mailboxes is stored in SECURITY TAB.
I need to list them badly :/