a multiple select list creates an array of the selections that where made.
the default template has code in place to convert the array to a comma seperated list using the implode() function.
you will need to edit:
<?php echo((isset($_POST["Contact_Us_group_Product[]"]))?$_POST["Contact_Us_group_Product[]"]:"") ?>
to:
<?php echo((isset($_POST["Contact_Us_group_Product"]))?implode(", ", $_POST["Contact_Us_group_Product"]):"") ?>
to convert the array.