If you want new registered user to be automatically subscribed to your jNews subscriber's list, use this plugin.
What is jNews?
jNews introduces you to the next generation of email marketing. A powerful newsletter component for Joomla 1.5, jNews gives you quality outputs with the newest professional yet coolest features ever making sure that your communications yields the best results that you really wanted.
jNews handles over a million subscribers efficiently, manages mailing lists orderly, and sends newsletters specially made for your customers.
Functionality of the plugin:
*Mailing Lists appear on your registration form
*Allows register to select the mailing lists they want to subscribe
*Automatically add the user to the subscriber's list after registration