OS X Lion: Activate a guest user account
Every Mac comes with a guest account, which you can activate when you want others to be able to temporarily use your computer.
Guest users can log in to the computer without using a password, but they can’t make changes to other user accounts or computer settings and they can’t log in remotely.
Allow guests to log in to this computer
Select this checkbox to let others log in to your computer without providing a password, but prevent them from logging in remotely.
If you select this option, you can also select the following one.
Enable parental controls
Select this checkbox to set up parental controls for guest users with log-in privileges. To set up the parental controls, click Open Parental Controls.
Allow guests to connect to shared folders
Select this checkbox to let a guest user access shared folders from another computer on your network.
You can also use a sharing-only account to give others remote access to shared folders.