You're gonna have to modify \templates\default\island_whos_online.tpl to change it up, and possibly the \cache_builders\online_now.php file to have it do an "add" up of all of them if it's not already in there.
So, depending on if you wanted to count spiders in your total or not, you could create a new variable called $tot_on and add up $reg, $anon, $users, $invisible and $spi if you want to count spiders or leave it out if you don't. (that'd be in the online_now.php
