One of the least attractive qualities of the new Facebook Groups is that anyone can create a group, and then they can add anyone to the group, so suddenly you are a member of a group for which you never asked to be signed up. This is a form of spam, pure and simple, and it is evil for someone to do it to you, just as it is evil for Facebook to set it up that way. Here is how to delete yourself from that group, and also how to report it as spam for signing you up in the first place.