Not sure why they are members, but probably something to do with phpBB's design. Google/yahoo/msn/whateverbots are search engine "spiders" crawling around the web indexing websites for the search engines. Its how the search engines work. They index millions of web pages using these spiders (just a bit of software running on a server somewhere) and store them in a huge database of sorts.
Ok, I'm sure its not exactly like that (I really should know this!) but you get the idea
All the search engines have their own bots that trawl the internet indexing it so as to make searches easier. To have our forum searched we have to have them allowed on as users so they are able to see it. There are certains parts that they can't see of course. As they are users they therefore show up in the users list. When I get a mo I'll edit the code so these bots don't show up.