Sneakernet

Create an OU separately and add the 20 users whom you want to give access the USB and applied the enable USB Policy on them and for the rest of the user Disable.

If you want to know more details about how to create group policy Just let me know exactly to how many users you want to give those permissions i will send you the Map structure of those policies.

Rizin

The users are all theoretical (user1, user2, .... to 20).

my question would be, how would you keep a user from plugging in another thumb drive that you didn't assign him/her or otherwise "authorize"?

The following two apps claim to have the ability to do it, the first one has a freeware version.

www.netwrix.com/usb_blocker_freeware.html
www.everstrike.com/usbsecurity/

However, I've never tried those.

Doesn't Languard have a facility in it to let you do something like that? Can't remember, but maybe the Bishop can refresh my memory

Yes GFI sell a tool that does it, GFI Endpoint Manager I think