All Magento models have a "getData" method available, which will return an php-array of key/value pairs. Try this at the top of your grouped.phtml file (after $_product is defined)
print('<pre>');print_r($_product->getData());print('</pre>');
You should see output that looks something like the following.
Array
(
[store_id] => 1
[entity_id] => 3437
[entity_type_id] => 4
[attribute_set_id] => 27
[type_id] => grouped
[sku] =>
[category_ids] =>
[created_at] => 2009-04-16 03:37:51
...
So, you can grab an array of properties and just pull the key out. You could also use Magento's convenience/magic getX and setX methods. On all Magento models, you can access any property in the data array by calling a method based on the camel case version of the name,
$created_at = $_product->getCreatedAt();
$_product->setCreatedAt($date);
So, whatever your custom attribute name is, you should be able to get at it using the above, and if you're not sure just print_r or var_dump the contents of the array returned by getData().
Finally, if the custom attribute is on one of the related products simple product, you'll wants something more like
$_associatedProducts[0]->getCreatedAt();