Here is a simple trick to get the sub-products of a grouped product on the category products listing page. Add the following code to the products list.phtml file.
$_associatedProducts = array();
if ($_product->getTypeId() == 'grouped')
$_associatedProducts = $_product->getTypeInstance(true)->getAssociatedProducts($_product);
Once you collect all simple product data associated with the grouped product, you can loop through the array and display the simple product’s data.