User Activity

I'm setting up a shop with a dozen Pixma printers and 2 dozen users. How do I configure so each user can choose any printer?
I'm trying to connect 4 pixma 6820s to 1 PC via a USB hub. I can connect 1, but connecting the 2nd knocks the 1st offline. I've tried with the 1st on and also off. Any suggestions would be gratefully accepted!