Live friend tracking allows you to track the location of your friends using the Cartograph live tracking service.
You can add a friend manually in the live tracking manager by going to the “Friends” tab and press the “+” button. A dialog will popup where you can paste your friend’s id.
As an alternative you can also create clickable links:
Friend tracking can be activated/deactivated by pressing the “Friends” button (a highlighted button means that tracking is active):