By navigating to Messages> My Subscriptions, users with read access can subscribe to a discussion, which means they will be notified when a message is posted to that discussion. The subscribe to a message board you can click Messages> Message Board and then click into a board itself and click the blue "Subscribe" button in the upper right-hand corner.

Service Planning allows security level 3 Planners to set the minimum security level permitted to create new discussions.  This setting can be modified on the Church Account Setting page (Account Church Account Settings).