If I have a guest can they see my computers?

It depends on which base station model you have. Click here for more information.

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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.

davefromwinnipeg wrote:

... 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.

About the Guest network feature of AirPort Extreme (3rd Generation or later), Time Capsule (2nd Generation or later), and AirPort Express 802.11n (2nd Generation)