It's possible that someone wanted to suscribe to that group and mistakenly entered your address.
It's also possible that someone though you would apreciate the suscription and gave you one.
You don't need to have a Yahoo! acount to suscribe to any yahoo discution group.
You can safely send a remove message. That message will only reveal your e-mail address and IP address. The group administrator already have your e-mail, and the IP address normaly change periodicaly unless you have a static one (a rare case). You don't need to state any justification, just asking to be removed is enough.
Electro, August 2011