TFS 2012 - Code review - Reviewer name is not in the list of allowed reviewers

You need to go to your Team Project and add some users to your Team. If you first select your Team Project \ Team you will notice that you have a "Members (0)" on the right. If you click "Manage all members" under that heading you can add any number of Windows or Microsoft Id users to that list.

Now that you have the users added to the Team, if you also select that Team in Visual Studio you will be able to select any of those users for review. This list is in fact of all users that have 'contributor' permissions to the Team Project which is granted by using the above process.

If you are having trouble setting up and configuring teams I have created a post on Modelling Teams in Team Foundation Server 2013.