While there are probably a lot of "lurkers" that aren't members, many of those people listed as "guests" could likely be members looking over the site while not being logged in.
Some of them could even be what I call "meathogs" ... simply looking for someone's report & pics that give up specific locations or other info they can use to horn in on the action, without regard to being a contributing member or even an ethical sportsman. They're one of the reasons it's suggested that reports contain "general info", and pics are not taken with the area or bank/shoreline in the background.