Adding user groups from a remote domain server to permissions of a remote desktop terminal server fails. why?

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http://serverfault.com – I have 3 computers, two of which are servers running Windows Server 2008 and another running Windows 7. One of the servers has the following roles installed; Active Directory, DHCP and DNS. The other server has a Terminal Server role installed. I am trying to log-on to the Terminal Server via Remote Desktop using the Windows 7 machine with credentials from the Active Directory server. Sounds simple enough, right? Well, no. Whenever I try to add users or groups from the Active Directory Domain server to the Terminal Server's permissions for RDP it seems to ignore, or forget, them. Though the (HowTos)