This project is not scheduled for development. Unfortunately we don't have unlimited development resources.
In order to materialize this project, could you please provide small details how you see it.
Scenario 1: Someone creates a group, and other contributors join to it (if the owner approves), later any member of the group can create a discussion which will be visible to all group members.
Scenario 2: Someone creates a discussion and other contributors have to request a permission to join this discussion from the discussion owner.