depending on how your site is set up, a new user should automatically be added to the "Subscribers" security role. this is an "Opt-Out" role for members who do or don't want to receive information from your website.

Using the newsletter module, just check / tick the "Subscribers" check box and send your news letter. Everyone who is a member of your website that has not opted-out of the role will receive your email.

Alternatively you can create a new security role specifically for receiving newsletters. Just make sure to tick the auto check box. This will automatically assign a new registered user the Newsletter role.

Hope this helps