No, no clue at all, actually. That listing in the admin area doesn't show just admins, it should show all users. That's also using a standard Joomla "User" button to create that modal.
Can you go to the Joomla Article manager and edit any article on your site. In the right side, there should be a similar "User" button for the author of the article. Test if that works, as it will tell us a lot about what may be wrong. If it does work there, can you let me know if you're using a different admin template than the standard Bluestork (in J2.5)?
Thanks,
Alex