Ad Domain Users to power Users

I'm having troubles adding Domain Users to the power users group. It worked for the first 10 or so users and now I can't add anymore. This what I do: Right click "My Computer" --> Manage --> Local users and groups --> Groups, select power users --> add --> Look In: Domain, then I select "Domain Users". Every time I do this I get an error message - “Processing the object domain users failed with the following error: The specified domain either does not exist or could not be contacted.”

Any Ideas
Thanks
Allan