Google Apps Premier Edition includes support for domain shared contacts.  Sometimes referred to as a global address list, this feature allows users on your domain to collaborate with other users both from within and without your domain.  This feature is commonly used to manage a centralized list of partners, customers and/or suppliers.  The email addresses added to your shared contacts will be available through autocomplete; when you're entering a recipient in an email message, you'll see your personal contacts suggested first, followed by entries from your domain shared contacts list.

As the domain administrator, you'll have full control over the domain shared contacts list through the domain shared contacts sample script on Google Code.

Because you cannot currently manage domain shared contacts through the control panel interface, you may wish to take advantage of our Shared Contacts setup service to perform this task for you.  All you need to do is supply us with a CSV file (we'll supply you with a sample) containing the contact information you wish to share within your domain.