Is there module that will add a drop down widget that auto-populates with a list of the site users?
You're looking for the References  module 


  This project provides D7 versions of the 'node_reference' and 'user_reference' field types, that were part of the CCK package in D6, at functional parity with the D6 counterparts. See http://drupal.org/node/533222 for details.


Similar module is Entity Reference 


  Provides a field type that can reference arbitrary entities:


You can add a new field for user list by selecting User Reference as a field type and select list as Widget Type in your content type.

Here is the screenshot for that