How to change search possibilities in assigning tasks to a user?

Summary

I would like to change the search view in 'assigned to' in tasks section.

Content

When you want to assign a task to somebody (in Candidate tab), you can do this via a field called 'assigned to'. When you click on 'add' you get a pop up window. Which page or list view is this? I can't find it, and I need to change the search criteria possibilities on this page.