pewc_get_products_for_cats_args.php

<?php
/**
 * Include grouped products in Products Categories child products
 */
function demo_get_products_for_cats_args( $args ) {
  $args['type'] = array( 'simple', 'variable', 'grouped' );
  return $args;
}
add_filter( 'pewc_get_products_for_cats_args', 'demo_get_products_for_cats_args' );