HOW TO: Task List - Assigned To - List

When you create a task list on a subsite, any WSS user that accesses the subsite is added to the "Assigned To" list.

HOW TO : Prevent other unwanted site collection users from showing up in the "Assigned To" list

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  Set unique permissions when you create a subsite and then add your users to that subsite.
  


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  Once the unique subsite users logon to the subsite, they will then be added to the "Assigned To" list.
  


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  Prevents other unwanted site collection users from being added to the subsite task list.
  

From MSD2D