I had this issue with converting Visual SourceSafe to Team Foundation Server 2008. Hopefully it will help someone out!
Had this issue and this is what we did to fix it:-
1. Pull out the users with the errors beside them.
2. In Source Control Explorer select the Client / Project.
3. Go into Team, Team Foundation Settings, Group membership, Team Foundation Valid Users, Contributors (double-click or select properties to drill down).
4 Add a Windows user & select the Users with errors from Active Directory.
5. Exit out clicking OK.
6. Rerun the import and you should be good to go!
I suggest using this tool as it makes life so much simpler!