TFS 2013 adding tester to a project

I would like to ask a little help to Team Foundation Server 2013. I have created a test plan with test cases and I would like to add a user to this plan as a tester but she does not display in the Assign list of Testing Center. Where can I add this user and what role to the TFS project, please?

I want this user to test the project in Visual Studio Team Foundation Server 2013 web app.