Dear Onesong:
With respect to any legal ground the group may have legally, it all depends. Since I don't know what was sent I am unable to advise if there is a legal authority that they can pursue you under. It also depends on which state you reside in. Each state has different codes. What was the manner in which you sent it; how did you obtain names and addresses; was it a mass e-mailing (which can fall under "spam" and thus, may be subject to spam laws in your state); were the addressees personal information (i.e., full names, addresses, e-mail addresses, etc.) shown? Can you give me specifics, as I may be able to advise as to the consequences of your actions.