In my experience, the problem lies with the plugin "BuddyPress Group Email Subscription" new users, not new groups settings : new users settings are never initialized to the defaults, so new users are not subscribed to the events they should receive.
For example, $ass_replies_after_me_topic is always empty if you don't go to each user's account setings for email alerts and click on the "Save settings" button.
I think new users should automatically be setup with the default settings that show on this screen, and so should all existing users upon plugin installation. This is not the case right now, so no default settins is taken into account out of the box, and nobody ever receives notification. This has conducted users to say the plugin does not work at all.
(Sorry this has been posted in the "BP Group Organizer" plugin forum, but the problem I describe concerns another plugin, http://wordpress.org/plugins/buddypress-group-email-subscription/, to which the original topic message makes a reference)