“Guests” are probably unregistered lurkers, although there seem to be a lot less of them than I thought. In fact the whole community seems smaller than what I imagined, but I like that. Much homier.
And if the version of vB here has the same features as we are running over at MousePlanet, then not only can it tell us how many people are currently here but also what the all time maximum was.
Probably a good thing that option is turned off or we would have people trying to arrange record breaking numbers.
I believe that when the SDMB made the Great Change from ubb to vBulletin, they turned on the option of showing which users were logged on. It only lasted for a day or so, because some users expressed concerns about their privacy.