If you ever look at "who's online" most of the people online are "guests". So I think they are people just looking at the site and browsing, but for some reason don't feel comfortable joining yet. Also, when you look at who is viewing comments, it might say "2 members and 12 guests" or something like that and it's happening in real-time. That seems to indicate that many people are just looking at comments and don't feel comfortable joining the conversations just yet. I am trying to encourage people to join so they can participate more, because reading about others that struggle just like us is one thing, but communicating with them is another step and can be very helpful, at least in my experience.