If you see ‘TF215085 local build agent not authorized to communicate with team foundation server’ when you queue a new build via Visual Studio, you would assume you just have to add the TFSBuild user to the team projects build group as the rest of the message suggests (right click in team explorer for the project and look at the group membership).

However this is not enough it seems if you are running a workgroup edition of Team Foundation server.  You also have to make sure the TFSBuild user is in the licensed users group for the team server (right click the server itself in the team explorer).

Now this could be an issue if you are pushed for users licenses, I suppose the answer is to use a normal user as the build agent’s login user.