WORKAROUND:
Here is a solution for adding users to TFS 2010 when your development machine doesn't have access to the remote domain.
Create the user on the server if you haven't already done so and Add the user to the ProjectTeam group.
Then on your local machine that isn't tied to the domain, simply create a user with the same username.
Right-Click on your project in Team Explorer and choose "Team Project Settings" --> "Group Membership..."
Click on the group you want to add the user to, and press the "Properties" button.
Select "Windows User or Group" and click "Add"...then just type in the username (you shouldn't need to specify the domain) - even if it initially identifies your current computer/domain...it should automatically change it to the remote domain.
Then you can delete that username from your local computer.
I spent about 4 hours trying to add a user for a remote domain before getting this to work.
Microsoft really should have thought about this scenario for remote developers, TFS running on remotely hosted servers and so on.
I hope that helps.