The user roster is available to all network users on the network.  People within your organization can use the Users to connect with others inside your organization.

External users are users outside your organization, and so this tab will not appear for them.  This is one of the many reasons why it is fundamentally important for PBworks Admins to set up external and internal users accurately.  If you are regular employee of a company but are designated as a "External", that will make the experience with PBworks fundamentally different and you will have much less access to key collaborative features.