When creating users for NPrinting you fill some of the following fields (see screenshot).
NPrinting will check against AD whether Domain Account exists and will automaticly authenticate users when they will try to login to NPrinting console or NewStand.
Be aware that if you will try to use recipient import wrong Domain account value will fail your import task!
You can also go to Admin-->Settings and change following setting to "Windows only", but to be honest i would keep NPrinting authentication as well just in case you do somethinng wrong with recipient import. You do not want get yourself locked out.