<?php echo $this->htmlEscape($_item->getShortDescription()) ?>

if($_product->isGrouped()) {
    $_associatedProducts = $_product->getTypeInstance(true)->getAssociatedProducts($_product);
    foreach($_associatedProducts as $_associatedProduct) {
        if($_description = $_associatedProduct->getShortDescription()) {
            echo $this->helper('catalog/output')->productAttribute($_associatedProduct, $_description, 'short_description');
        }
    }
}