This particular group was about serial killers, etc. As I have this listed in my user info, you can assume it's a subject I get a kick out of. This person, in the 'welcome' mail, claimed they had seen my comments on LJ, and thought I'd be an asset to their group. I, however, have not been in a convo concerning this topic in a long time. I can only assume that they saw that in my user interests, and went form there, so who knows who else might have been signed up.
This was not an invite to join...this person actually SIGNED ME UP. Yahoo Groups allows this, with the disclaimer to 'use this feature wisely'. Since it isn't against the Terms of Service to do it, however, I have mentioned it to Yahoo abuse.
Just a word of warning to the world.