On the “for” attribute some people may think that “[project]” is a place holder for the project's name, but it is meant as a literal.
If your Team Project is named TheGreatestProjectEver, you should use "[project]\AllTesters" literally as stated above in documentation and not " TheGreatestProjectEver \AllTesters"
Using "TheGreatestProjectEver \AllTesters" will return:
“Warning: TF26171: (User/group {0} is not found.
TF26204: The account you entered is not recognized. Contact your Team Foundation Server administrator to add your account.”