TFS 2013 - Limiting Users List in User DropDown

Below is an example of how to limit users in a user selection dropdown:

<field name="Assigned To" refname="System.AssignedTo" type="String" syncnamechanges="true" reportable="dimension">
   <!-- show users that are part of the team only -->
   <validuser group="[Project]\Contributors" />

   <!-- below is another way to do that
   <allowedvalues expanditems="true">
      <listitem value="[Project]\Contributors" />
   </allowedvalues> -->

   <default from="currentuser" />   <!-- this one to have the logged on user to be automatically selected by default -->
</field>

Modifying and Creating Work Item Type in TFS 2013

To modify a work item type in TFS 2013, we need to these steps:
- export the work item type xml file to our machines
- modify that file
- then import back the modified xml file to the TFS server

To create a new work item type, we need to:
- create a new work item type xml file
- import the new xml file to TFS server

We use Visual Studio Command Prompt to do this.

To export work item type, run this command:

witadmin exportwitd /collection:[team project collection URI] /p:[project_name] /n:[work item type name] /f:[destination file]

examples:

C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE>witadmin exportwitd 
/collection:"http://myTFSserver/tfs/DefaultCollection" /p:MyProject /n:Bug /f:C:\Users\me\Desktop\bug.xml

C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE>witadmin exportwitd 
/collection:"http://myTFSserver/tfs/DefaultCollection" /p:MyProject /n:"Product Backlog Item" /f:c:\Users\me\Desktop\pib.xml

To import work item type, run this command:

witadmin importwitd /collection:[team project collection URI] /p:[project_name] /f:[source file]

example:

C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE>witadmin importwitd 
/collection:"http://myTFSserver/tfs/DefaultCollection" /p:MyProject /f:c:\Users\me\Desktop\task.xml

To delete work item type:

witadmin destroywitd /collection:[team project collection URI] /p:[project_name] /n:[work item type name]

example:

witadmin destroywitd /collection:"http://myTFSserver/tfs/DefaultCollection" /p:"My Project" /n:"Impediment"

To rename work item type:

witadmin renamewitd /collection:[team project collection URI] /p:[project_name] /n:[existing work item type name] 
/new:[new work item type name]

example:

C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE>witadmin renamewitd 
/collection:"http://myTFSserver/tfs/DefaultCollection" /"My Project" /n:"Product Backlog Item" /new:"Enhancement"
Are you sure you want to rename the work item type Product Backlog Item to the new name of Enhancement? (Yes/No) Yes
The work item type was renamed. 

To export process configuration:

witadmin exportprocessconfig /collection:[team project collection URI] /p:[project_name] /f:[destination file]

example:

C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE>witadmin exportprocessconfig 
/collection:"http://myTFSserver/tfs/DefaultCollection" /p:MyProject /f:c:/Users\me\Desktop\ProcessConfiguration.xml

To import process configuration:

witadmin importprocessconfig /collection:[team project collection URI] /p:[project_name] /f:[source file]

C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE>witadmin importprocessconfig  
/collection:"http://myTFSserver/tfs/DefaultCollection" /p:MyProject /f:c:/Users\me\Desktop\ProcessConfiguration.xml

Reference:
MSDN: Import, export, and manage work item types [witadmin]