It would appear that the file you would need to alter is yourmoodle/admin/user.php As usual, I do not recommend altering core code as you have no idea what else your changes might affect.
Instead of altering core files, I would recommend that you install and use the plugin, Ad-hoc database queries. If you look through the ad-hoc contributed reports, there is a good chance you will find find a report that gives you what you want, or one that is so close, that it only needs minor modifications.
You could also try using the Configurable Report plugin. I have always used Ad-hoc, so don't know much about this one and if it has any pre-made reports you could use.