Problem: You're working on a system that's joined to a domain, but when you try to add domain resources to security, local groups, etc., the only Location showing in the dialog box is your local system -- no domain. You may even be able to log on to the system with a domain account, but you're still limited to local resources when dealing with the Add thingie.
Possible Solution: Double-check where your DNS server settings are pointing to. In my case, I was working on adding a MOSS VM to the Exchange VPC I downloaded from Microsoft. The Exchange is set up as the domain controller and has AD and DNS running on it. However, I'd had it grab an IP address from my local DHCP server -- and as a result both the Exchange VPC and MOSS VM were using DNS servers other than the Exchange VPC. Some domain-related calls worked fine, like joining the domain and logging on with a domain account, however, others didn't.
I put manual DNS server entries in the network connection dialog for each system and put the Exchange VPC's address as primary DNS and my network's DNS server as the secondary. Now everything's hunky dory. Domain resources are now available when I'm altering group membership.