How do I find my WEP key please?

What type of security do you have enabled for your network? You would have to provide the password you created to access your network, which is dependent upon the type of security you have enabled. For Mac networks that's typically WPA2 Personal. Your guest would need to configure his/her network to use the same security method.

Your password might be stored in your Keychain if your Mac connects to your network via Wi-Fi. However, as Kappy said, whether your guest can connect to your network will depend on whether they are using the same type of security as you are.

If your Mac does connect to your network *via Wi-Fi* then access your Keychain by the following:
Open Finder, navigate to Applications, then Utilities, then open 'Keychain Access'. In the Keychains sidebar, click on 'System', find the name of your network and double-click on it. At the bottom of the window that appears should be an option to "Show password'. Tick this and enter the password for your mac user account when prompted. The password that appears in the box should be the password for your Wi-Fi.

Holygemoly.