How to add agents to support system plugin

I have 8 part time assistants that work with me but I cannot assign them tickets. What user rights do they need so I can assign them or they can self-assign themselves to a ticket? Is there an easy way without my playing with the code (and thus causing issues with future upgrades) to change how closed tickets appear. It would be nice to have options to show/not show closed tickets or set a drop date that after X number of days the closed tickets no longer display on the front end but could be accessed from the back. And while I'm asking how about an option to clear all closed tickets before a certain date? This system could get unwieldy if we get a couple tickets a day for a year.

Thanks!

Michael