At the moment, once you are logged in and viewing a map in AGOL, there is no easy way to log out again. The only way to do so is to go manually to the organization main page, where the user login details are available, and click the down arrow to get to log out.
A logout link would be very useful for those of us who use different credentials, and I would like at least the option to add this to a web map or web app, or to have a logout widget.