The data you are trying to get is inside an array that is inside an array.
Array( [kit_id] => 570 [module] => uc_product_kit [attributes] => Array( [Attending] => Array( [0] => Jane Doe, John Doe ) [Note] => Array( [0] => We are friends of Eric Smith and Sandy Smith ) ) [shippable] => 0 [unique_id] => 4c7d632a8090c7.95333579 )
You need to add the name of the array that this array is in.
$data = unserialize($output); print $data['attributes']['Attending'][0];
Little rusty at this. Hope that helps. Cory