Drupal Form Suffix + Suffix As a href + pop up the drupal users listing with checkboxes by click on that link
What you need is pretty much a huge select box :)

i can think to a couple of different approaches, but to mention one, i would proceed like this


Create your form and set the default values obviously.
At the end of your form add a  that will wrap around the list of possible choices
Target that  with the show/hide script triggered by the click on the link.
Create the list, using either a table or several divs, as long as you give each row an #id, something like 
user-$uid".
since you are here you can even set a class for the rows that will highlights the row on hover.
Split each row into, assuming tables, cells and populate the table with the user data you need.
give each cell the same #id of its row, extend with something else to obtain things like

"user-$uid-mail"
"user-$uid-name"
I would avoid the checkboxes and go with an onClick event on each row, that calls a function passing the row #id as parameter.
The function will, hide the list, use the row #id to read the values of the fields, like

document.getElementById("user-$uid-name").value


and write the values inside the form elements.

profit.
i may have typed some function wrong, but that's the idea