I see that there is currently one person online and registered who is listed as "invisible". What does that mean?

I believe this is done by going to the top bar, go My Stuff/My Preferences look to the last option "Hide from Who's Online?" and choosing Yes. It's my guess that it's so a person can still be registered, read the forums, and no one will know who it is.

That's my best guess though, I could be wrong.