Unzip it into your moodle folder, and it should add the files admin/report/userroles/index.php and one other.
It is not designed to work efficiently on huge sites, (it really needs to be made to print a paging bar if there are too many results returned), but it should be OK up to a few thousand users with a particular role.
So, once it is in a better state, I will of course, put it in contrib and the MnP database.
I did this on my own instance, not the production server, but I think the first problem I had was that question/upgrade.php did not exist. I must have an older version of Moodle installed.
So then I upgraded my moodle instance to the latest, downloaded today (Aug 25 2007) and get these errors (which, of course, make no sense to me!) I surely would like to have this report, so perhaps some of this will help you. I am using the Moodle4Windows install, so that is WAMP.
Warning: Missing argument 2 for admin_externalpage_setup(), called in D:\moms_stuff\moodle_18\moodle\admin\report\userroles\index.php on line 16 and defined in D:\moodle_18\moodle\lib\adminlib.php on line 2603
Notice: Undefined variable: adminroot in D:\moodle_18\moodle\lib\adminlib.php on line 2616
Fatal error: Call to a member function locate() on a non-object in D:\moodle_18\moodle\lib\adminlib.php on line 2616
Did you manage to te this hack working? I've installed it on my site (1.8.2), and although under the "reports" menu, I get the option to click on userroles, I get a blank page on my local test machine OR HP 500 (Website cannot display the page) message.