If one member of your team has checked out the file without checking in and you want to edit the file, you can check the file in by this way:
C:\Program Files\Microsoft Visual Studio 8\VC>tf undo /workspace:UWORKSPACE;USER /se
rver:USERVER $FILEPATH
And pay attention plz, the change of the file by the member will not be save at the server.
Also you should be a administrator of the system.