Typically there are a fair few guests on forums. People do a Google search for something like "Bipolar Disorder" or "OCD" and voila, they find PsychForums, which is fairly highly ranked in Google's indexing system. They then come here to find information for themselves or someone else about a psychological situation. It takes more effort and desire to actually sign up as a member and often people don't need to go as far as to even sign up to get the info they're looking for.