Guests are those reading the forum without being logged in.

Any half-decent forum software makes the guest number available. I bet that's the number on internet connections minus logged in users.

Amy?

I'm guessing the "guests" are comprised of actual guests plus search engine spiders, etc.

And I believe some forum software has the option of exaggerating the number of guests - don't know if that happens on this forum, though.

Corrected. Not by a search engine spider :D

Correct, in vB3 "guests" are real visitors + search engine spiders. The numbers here don't appear to be inflated, I believe that's the real number.

Also, https doesn't add meaningful overhead, and over time implementing it will continue to help with rankings too.