Windows small business server 2003

Windows smal business server 2003 domain authentication
I hate to sound stupid but here goes-
I am running 2 servers both with Windows Small business server 2003. 1 is the primary domain controller, the other is a file server.
When creating a share on the file server I want only a specific security group from the domain controller to have access. When I try to add the group to the permissions for the folder the machine cannot find it. yes both servers are on the domain, and the file server is authenticating users at logon any help would be greatly appreciated.