Its actually quite easy to do this, open up TPsubs.template.php
find:
'.$txt['tp-guests'].': '.$online['guests'].'<br />
after add:
'.$txt['tp-spiders'].': '.$online['num_spiders'].'<br />
you will need to enter a new text string tp-spiders to your language file, open languages/TPortal.english
Find:
$txt['tp-guests'] = 'Guests';
after add:
$txt['tp-spiders']='Spiders';