This system allows for private groups to be created, which can contain private articles and discussions. The system administrators control membership of these groups. If you are member of a group then you can read and post within that group, otherwise the existence of the group will not be visible to you.
Groups you are a member of will appear on the groups listing. These let you access the home page for each group, which will list the articles visible to that group only.
In addition, any private posts relating to any private group you are a member of will appear on the main "what's new" page, marked 'private'. These will not be visible to non-members of the group, and neither will any of the group displays be available.