First of all, use the last version of CUPS!
Create in the PC that will act as Print Server, the user that you want to permite use the printer
1) The printer must be installed in your system
3) Click on Share printers connected to this system and save the changes
4) Click on “Printers”
5) Click on the printer you want to manages
6) Click on the right arrow of the “Administraction” and choose “Set Default Options”
7) Click on “Policies” and choose “Authenticated” in “Operation Policies” & save changes
8) Click on “Printers” again
9) Click on the right arrow of the “Administraction” and choose “Set Allowed User”
10) In the blank line write the users that you allow to use the printer, separated with space or comma
11) Choose “Allow these users to print” and save the changes
In the client PC, add the printer via LPD using the IP of the server, if it change the IP with teh name of that PC, change again to the IP to avoid problems, there you can choose the “Print queeu”, choose the one that you share.
When a user print something, will ask for user/pass