You can turn on “fast user switching” to allow more than one user to stay logged in to a computer at a time.
When fast user switching is turned on, the name of the current user is displayed in the upper-right corner of the menu bar.
You can click the name to select another user to switch to.

If the other user’s account doesn’t require a password, the computer switches automatically to the new user’s home folder. Otherwise, you see a dialog asking for the user’s password.
And now, howto get this feature:

Step 1:
Open System Preferences

Step 2:
Click Accounts

Step 3:
Auth yourself as admin if some of the boxes and/or fields are dimmed out.

Step 4:
Select Login Options (under the account-list)

Step 5:
Select “Enable fast user switching”

Like always, best regards
fidel

apple informations to this topic: here

Tags: enable fast user switching, howto