To get the author/modifier id:

$author_id = $content_genius->info("article_created_by");

or

$author_id = $content_genius->info("article_modified_by");

The contact id is going to be different to this I guess. But do you need the contact id or is the user id enough? It's probably just a quick database lookup to convert between the two things.

$author_id = (int)$content_genius->info("article_created_by");
if ($author_id > 0) {
  $query = "select id from #_" . "_contact_details where user_id = $author_id";
  $db->setQuery($query);
  $contact_id = $db->loadResult();
}

Or if you want to get all the other info from the contact record:

$author_id = (int)$content_genius->info("article_created_by");
if ($author_id > 0) {
  $query = "select * from #_" . "_contact_details where user_id = $author_id";
  $db->setQuery($query);
  $row = $db->loadAssoc();
  if ($row != null) {
    echo $row['name'] . '<br />';
    echo $row['address'] . '<br />';
    echo $row['suburb'] . '<br />';
    echo $row['state'] . '<br />';
    echo $row['country'] . '<br />';
    echo $row['postcode'] . '<br />';
    echo $row['telephone'] . '<br />';
    echo $row['mobile'] . '<br />';
  }
}