Was the notify really from yahoo? I get often some mails from amazon, yahoo and even about a problem with my credit card (I never had one ;o). Did you click on the link on the mail? In most of the cases it does not lead to the real amazon (or whatever) page but to some criminlas you wanna pishing your password.
When I move my cursor over a link in an email I can see where it will lead me to and this is often something complete different that what is written in the text!
Think this persons have entered your account. Maybe you can change the password again or you must close the account :o(
Welcome to my world. I finally closed my account after I have sent numberous reports of issues with no responses back. This was with my e-mail though not my groups account. Can you do an I forgot my password with it? If so I would try that. Also if you can contac tthe group monitor to let her know you are having an issue and creating a new account they will work with you. Good Luck!!