add_filter( 'wp_insert_post_empty_content', 'test_user_can_do_this', PHP_INT_MAX -1, 2);

function test_user_can_do_this($maybe_empty, $postarr) {
   // not a special role, ignore
   if (!is_client_user()) return $maybe_empty;
   $post_owner = $postarr['post_author'];

   // custom tests compare post owner vs current user
   if (is_grouped_with($post_owner)) return $maybe_empty;
   if (is_grouped_under($post_owner)) return $maybe_empty;

   // otherwise, deny save
    return true;
}