The only way to do this would be to prepopulate the contact lists of each user with all known SIP-enable objects in your address book. It's a manual process (search on OCSCM and LCSAddContacts.wsf) and any changes in your population (new/removed accounts) would not appear until added again. There isn't a way to dynamically add all.
One workaround would be to create a distribution group in AD that contains all of your SIP-enabled OCS users and then maintain that single list. User's can add that group to their client and then expand it to see the names of all the users.Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS
You may use (OCSCM) Microsoft Office Communications Server Contact Manager.It's an administration mmc tool that will integrate with Microsoft Office Communication Server 2007 and Active Directory to provide an easy way to provision OCS users’ contacts list. OCSCM is designed to show you how to manage the contacts and groups for individual ocs users by interacting with the Active Directory Domain Services and Microsoft Office Communication Server 2007/R2 and lcs 2005.