Just is case…
In your first post you listed your add_action code as:
add_action( 'groups_new_form_topic_post', 'my_notification_new_post' );
If that is the actual code you’re using, then you have a typo. You need to change the word “form” to “forum” to match the do_action line. Of course, you need to include the proper parameters as nicola pointed out.
I hope it’s as simple as that little typo!