...it isn't phishing email so don't worry, but it is something I accidentally did in Yahoo!. Feel free to ignore the email; in fact I've been told that for at least one person, the email automatically landed in their spam folder anyway, which is really funny and not a good sign for whatever Yahoo's going for with this new feature of theirs I know nothing about.
Haha... It landed in my junk, on Hotmail... which just so happens to have a competing service with what the invite is for... =P