They're called lurkers.
It is an officially Good Idea to lurk a while before you post, to get the feeling of the group and do some research into what has already been discussed to death.
Remember, our moderator did advice that people look into what was already available before posting? Well, it would seem that they're looking all right.
And since you don't have to be registered to lurk, only to post, the path of least resistance suggests that lurkers by far outnumber the regular posters.