Guests may be unregistered people who are browsing the forum but, looking at that list, I suspect many of them may be web crawlers in disguise (the Yahoo slurp spiders are a form of the latter):
There are Research companies and other 'search engines' that search using cloned information to make them look like normal 'guest' users. I came across Munax.com last night accessing my forum, apparently they go to great lengths to reveal why their spiders pretend to be normal browsers, it's mostly down to them wanting to see the same content as a human user. There is also the possibility that certain sites that do advertisement system require the whole user base to see the adverts and not be directed to a bots only page, this can be checked on by the advertisement company or third-parties on their behalf to see if their TOS/TOC's have been violated.
