The Global Address List, sometimes referred to as "GAL", is a dynamic list containing all users on your domain. The Global Address List can be managed by your domain administrator in order to hide users if desired.
Contact information for the Global Address List is pulled from individual profiles of users. To update your listing, go to the Settings menu and choose My Profile. The Global Address List functions in a way almost identically to My Contacts.
Please note that all information you enter into your profile can be seen by all other members of your domain.