I’ve changed my password since this started. It must be possible to add people somehow, I’ll have to check with the moderators that I know.
Well, sounds like something's been hacked, either on your end or on Yahoo. I can't imagine why a free Yahoo identity is all that valuable? Is it possible that someone is playing a dumb joke on you? If not, I wouldn't use that computer for logging into your banking accounts until you get it checked out for spyware, worms, or whatever.