If you want people to be able to add contacts to a central address list, then
the Global Address List is not the list you want to use. Instead, you can
create a public folder in the Public Folders\All Public Folders hierarchy to
hold contacts and set access permissions for that new folder appropriately.

You cannot, however, force Outlook to use only that public folder for
contacts. You'll need to educate your users in its importance. You might also
want to looik at the tools available for synchronizing public and mailbox
folders -- see http://www.slipstick.com/outlook/sync.htm