User data is not accessible meta data in that regard. One solution would be to create a list for use in a drop-down. You will have to update the list and the drop-down every time a new user is added. See our help guide for information about creating a list: /Documentation/manual/html/?fg_adding_a_list.htm.
You could also manage this with multiple roles. Assign different users to different names roles in the workflow.
You could also use Positions and change the owner of the position in the dashboard. Help guide: /Documentation/manual/html/?d_positions.htm
Either way, you will have to manually update as users are added.