Assign Project-Level Permission to New TFS Group

I've just upgraded to TFS 2012.3 and am trying to configure custom permissions for an existing TFS Group (UrbanGroup1). When you right-click on a team project and choose Team Project Settings>Security, it brings up web access in the browser. Here, it's possible to create a new TFS group and assign it the permissions you want, however you cannot use an existing group (UrbanGroup1 that's used on all projects). 

Any thoughts? I feel like there may be something that I'm missing.