Any idea how i can add an activity post notification when someone (friends only) gets tagged?
Photo tagging doesn’t create any activity when a user tag his friend but instead creates a BuddyPress notification. If you want to create BuddyPress activity, you will need to write some custom code for that. Check this hook
rtmedia_sent_new_tag_email for more details in RTMediaTagNotifications.php file under app directory.