Drupal 7, Entity Reference user autocomplete field
Set up a view for the autocomplete.

You can use this tutorial: http://speaksdrupal.com/blog/custom-entityreference-handlers-using-views

Then add that as the view to use for autocomplete.

http://screencast.com/t/8m0O7F8zV

Whatever fields are searched will show up in the drop down.  So if you remove e-mail from the list of fields searched, it won't show up in the drop-down.