TFS 2013 - change assigned to drop down list for work item bug, task etc

Using Team Foundation Server 2013 with Visual Studio 2013 and creating a work item like bug, task or issue the assigned to drop down was display far more people than where on my team.  As there is no obvious guid having an abortion to change who is listed then I did some digging and found it involves a small bit of effort which I will document here:-

I am using Visual Studio 2013 and Team Foundation Server 2013

You will need to download and install Microsoft Visual Studio Team Foundation Server 2013 Power Tools (please not if you already have a TFS Power Tools you will need to uninstall):-

Once installed, open Visual Studio 2013 and connect to your TFS 2013 Server.

In Visual Studio 2013, Menu -> Tools -> Process Editor -> Work Item Types -> Open WIT from Server

Select a Project and a Work Item Type e.g. here I will use Issue

A form will open in in Visual Studio like so:-

Find the Assigned To row (selected above).  Double click and you will see the Field Definition.  Select Rules Tab:-

Edit VALIDUSER row and you will see a VALIDUSER popup form:-

Now select a group you have all the users you want to display, here I am using the [Global]\Developers TFS Group

OK all open pop up forms and save the *.wit file.

Now you need to upload this file back to TFS.  To do this use almost the same menu opitions:-

In Visual Studio 2013, Menu -> Tools -> Process Editor -> Work Item Types -> Import WIT

Select the correct project and browse for the *.wit file you just saved.

Once uploaded try to create a new Issue for that project and you will see that only the users selected in your TFS Group are now displayed (if you are using the Web abortion procedures TFS version make sure you F5 to refresh otherwise you will not see the changes).