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