Apple, System Preferences, Sharing
check Internet Sharing
on the right, select something like
Share your connection from: Ethernet
to computers using: [X] Wi-Fi
click WiFi Options and give your network a name, go with WPA2 Personal security, and set a password. Use that information to connect from the guest computer.
(after connecting an iPhone or iPad, you will see that listed in with WiFi, you can share your ethernet with your phone or iPad while connected with the dock cable to really speed up app downloads)
