This module sends email notifications to members of a Drupal group when new content is posted to that group. It integrates with the Group Node module to provide this functionality.
Notify (email) members of a Group (https://www.drupal.org/project/group) when new group content is posted via Group Node plugins.
Once the module has been enabled, browse to Administration -> Groups -> Group types -> Set available content. Plugins provided by "Group Node" (gnode) can be enabled to send email notifications. Click "Install" or "Configure" and check "Notify group members" to enable.
If you experience a slowdown when saving a group node (lots of members to notify, perhaps) I suggest using the queue_mail module to mitigate this.