Invite other users to become a member in a specific group.
Procedure
The user receives a notification that they have been invited to join a private group. This user can then request membership of the group ( Find groups > Send request). If the group coordinator approves the request, the user can read the posts, comments, etc., by this group, and can himself post content in the group.
Group coordinators can add more users to a group directly (Add colleagues). That user immediately becomes a member of the group and receives the posts, comments, etc., by the group ( My feed).