You may download it from here.
About the tool (in a few words):
Since TFS use a lot of components and services (reports services, analysis services, TFSadministration control, SharePoint, TFS account service etc.) its being difficult to manage and maintain groups and users for all the components.
The "TFS Administration tool" help us to maintain users/groups permissions in one tool, after you define/edit/delete user or group it will automatically update all the services&components for you.