Add "email" to /admin/people overlay view
Have you tried using Views for this? This will work well if you want to make a new page that lists all of the users. It won't work out of the box if you want to add the email information to the existing admin/people overlay.


To use Views, enable the module and go to admin/structure/views/add
Give the View a name, and then for the 'Show:' option from the
dropdown select 'Users' 
To create a new page, leave the 'create a new page' item checked, give the new page a title and path, and under Display Format select 'Fields' Hit Continue and Edit 
On the next screen under Fields hit the Add button, and select User:email


There are tons of options to configure with Views - check out the Views Documentation for more information if you're new to Views.