title: Can i allow a user to modify a work item while denying him from create a new one
body: <p>I am setting up a tfs 2013 environnement,
  In a Scrum team project, we would like a few users to be able to create backlog items and tasks, but we would like that other users to only be able to change the task their are assigned. Or at least we would like that these users can modify existing tasks, while not being able to create new one, or move them to other sprints</p>
  
  <p>at this time, with the security parameters i have found, i can only either allow a user to do everything (create and move a backlog items from a sprint to another, modify it, delete it...), or nothing (if you can't create or move, you also can't modify an existing one...)</p>
  
  <p>any clue how i could proceed ?   </p>