Troubleshooting "Team Foundation Server cound not resolve the user or group"
Here's what to do when you're adding an Active Directory user to a Team Project and you run into this error:
Team Foundation server could not resolve the user or group (name). The user or group might be a member of a different domain, or the server might not have access to that domain. Verify the domain membership of the server and any domain trusts.
As documented in this forum thread, this error usually occurs when the client attempting to add a user to a Team Project doesn't have the same domain permissions as the Team Foundation Server.
The first thing to try is adding the user via the Team Foundation Server. Log in to the server and either use the tfssubscribe.exe command line tool, or if Visual Studio 2005 is installed on the server, add them that way. But make sore you do this on the server.
If adding the user works on the server, but doesn't work on the client, then you know it's due to the difference between client domain permissions and the TFS domain permissions.
UPDATE: Don't forget to check the credentials of the TFS service accounts. If the TFS service account password has expired, or if the TFS service account is otherwise invalid, you can see this error message as well.