To switch to a different user, first click the 
in the upper right corner of the screen. The Login window will pop up. Enter the password of the user that you would like to sign in as, and then press Enter or click OK. You will see the calendar change to the user's calendar.
The will be disabled while you are working in a screen within the system. To switch to a different user, exit the window by clicking the Exit button in the bottom toolbar, then follow the steps above.