They're not "members" or even actual people. They're spam bots, automated programs that scour the web looking for PHP based forums (like this) that meet certain criteria, such as easy registration. Once they find a suitable forum, a series of scripts runs that begin creating accounts and posting spam. In their own way, they are clever little programs. A royal pain in the tushy, but still clever.
Bots like the ones you've seen are the season that many sites now require you to type in a word that is displayed in an image before registering or posting. "Captcha's" are a way of adding a layer of security that ensures (or tries to ensure) that there is an actual person sitting at a computer looking at the page.
The downside to captchas is that the bigger spammers are now paying offshore companies to have people spend ten hours a day figuring entering the answer to captchas into a database (for about a buck a day) so that the scripts can take the image a verification page shows and create a mathematical "hash" of that image, then compare it to the database of known answers, and try registering using the answer it gets. The labor cost is cheap enough so that the spammers can still make money, since the advertising end of their business is essentially free.
Isn't the internet a wonderful place?

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Phil K.
Yankee Air MuseumSystems Admin / Ramp Crew / Professional Photo Ruiner