Cable Modem

we have a motorola cable modem installed. It has 2 interfaces USB and LAN. The computer is connected through USB cable.Modem doesnot required manual setting , it automatically get IP address from server(plug and play).
Now we want to share the internet; i have tried wingate proxy server but at clients end they receieve socket error although wingate gatekeeper shows clients computer name and IP address. Please help in this regard.
Also if i connect the modem with cat 6 cross cable to our network swtich but in this case all the users on network will be able to use the internet. I want specific users may use the internet.
How is this possible???