How to Switch Between Users on a MacBook
You can have several different user accounts on your MacBook. Each account can have its own desktop and preferences. This allows several people to share the MacBook and have it set up as they want. One user does not have to log out of the MacBook to switch to another account. With OS X's fast user switching, you can switch between user accounts on the fly.
Click the "Apple" menu, and select "System Preferences."
Click the "Lock" icon, and enter an administrator user name and password. Then click "Login Options."
Click "Show fast user switching menu as:" Select whatever display option you want.
Click the current user name in the menu bar when you want to switch accounts. Select a user account to switch to from the drop-down list.