What is the Global Address List / Address Book
The Global Address List (GAL) is a searchable directory in Outlook of all the users, groups, shared contacts, and resources defined for your Google Apps domain. These same global contacts are available from the Gmail interface on the web. By default, global addresses are shared with everyone in your organization and include each user’s primary email address and any email aliases or nicknames. The name format that's used (e.g., Last name, First name vs. First name, Last name) follows your name format setting in Gmail.
As a Google Apps account administrator you can: