Interesting. Sounds like a web crawler, but I have no idea how it can be registered as I have to approve every member. I just did two new members - Colin Johnson and Chris Wildey, but nobody else!
The bot is a crawler, the forum allows these to enter through a back door of sorts, this allows parts of the forum to be available for search on Google, this leads people to the forum, but they are unable to read/write without actually subscribing!
Every time I close the door on reality it comes in through the windows.