You can email the registered users on your site with a tool built in to Joomla. This can be used to notify of new features on your site or as a newsletter.
1. From your admin panel, go to Tools -> Mass Mail.
2. You can choose which group to send mail to. If you would like to mass mail to a group and it’s child groups, select Mail to Child Groups. You can also choose to change the formatting to HTML. Note that if this option is selected, there is a higher chance the email may be marked as spam by mail filters.
3. Enter a subject and the message body for the email.
4. Select Send Mail when finished and your email will be sent.