Checking our “Domain Admins” group with powershell I discover a strange behavior; running these 2 command I received 2 different results… some users are missing on first:
get-adgroup "domain admins" -property members | select -expandproperty members
Get-ADGroupMember "Domain Admins"
Investigating I discover that AD group members attribute is not populated with the user Primary Group. ADUC and Get-ADGroupMember are aware of that and show also Primary Group but LDAP query to members attribute not.
Check this: http://support.microsoft.com/en-us/kb/275523