As you use SharePoint Workspace, you build a list of people with whom you interact. In some cases, you build this list directly, for example by adding contacts to your personal list. In other cases, you build this list indirectly via your interactions with members of your workspaces. For example, when you join a workspace as an invited member, all other members of that workspace immediately become available to you for starting new activities. Similarly, any person who joins one of your workspaces (either by your invitation or some else's invitation) immediately becomes available to you. The aggregate list of all people you interact with via SharePoint Workspace is your list of Known Contacts.
On the File tab, click Info, click Messages and Contacts and then click Contact Manager. The list of displayed contacts is your Known Contacts list.