As a Moderator How do you set up a welcome letter to be sent automatically to new members?

My group is set up to allow anyone to become a member or quit being a member. I recieve emails with each new member, but how do I send out a welcome letter, to make each member feel a little more comfortable with the group. I don't know where to go to even start other then sending a letter each time I see some one has signed up. I'd rather they get one like right after they sign up auto matically.