From Yishay Mor for the OLDS MOOC.
Each group will have a cloud listing all group members.
The creator / owner of the group cloud will be able to define if it is open, by invitation, or people can ask to join. - It is possible to send messages to all group members. These messages are listed at the bottom of the goup cloud and delivered by mail (subject to user preferences).
The creator / owner of a group can define who can send messages to the group members (creator only / all members / anyone)