If you have an AirPort Extreme or the latest AirPort Express, you can enable a guest network with a seperate SSID (network name) and its users cannot see anything on your 'native' network.
Select your AirPort device, click EDIT, select Wireless, and check the box labled "Enable Guest Network" assign a unique network name and password. click update and you're all set.
... can I set a guest network so they can't see my computers
Yes, if your base station supports Guest Network.
Guest Network clients can communicate with each other if you allow this in AirPort Utility, but your computers will be inaccessible even if their Sharing Preferences are configured to allow file sharing.