303i

If you’re using a woothemes theme, just add it in the
“You can add custom functions below”
section of functions.php. Example:
http://puu.sh/7Cuee.png

Nope, didn’t work, pic of edit:
http://puu.sh/6H4j9.png
Could this be related to the undefined property error?

Just finally managed to fix, here it is if anyone wants to know:

/**
* Returns max price for grouped products
**/
function wc_grouped_price_html( $price, $product ) {
	$all_prices = array();

	foreach ( $product->get_children() as $child_id ) {
		$all_prices[] = get_post_meta( $child_id, '_price', true );
	}

	if ( ! empty( $all_prices ) ) {
		$max_price = max( $all_prices );
	} else {
		$max_price = '';
	}

	$price = '<span class="from">' . _x('From:', 'max_price', 'woocommerce') . ' </span>' . woocommerce_price( $max_price );

	return $price;
}
add_filter( 'woocommerce_grouped_price_html', 'wc_grouped_price_html', 10, 2 );