Steps to replicate:
1) Select a category
2) In the settings block, select 'Assign Roles'
3) Choose a role to assign
4) Try to search by typing some text into the search text field below the user list.

In IE8 this results in a javascript error complaining about an undefined object.

Offending line:
this.timeoutid = setTimeout(function(obj)

{obj.send_query(false)}, this.querydelay*1000, this);

This can be fixed by changing using YUI's 'later' function instead of setTimeout:
this.timeoutid = Y.later(this.querydelay*1000, e, function(obj){obj.send_query(false)}

, this);

Patch file attached.