Have your form use the following API to register the user and it will create them properly within CB and Joomla.
www.joomlapolis.com/documentation/279-community-builder/tutorials/18362-registering-a-user-through-cb-api
You can also use CB Auto Actions and let it handle all of the storage for you. To do this create an auto action leaving Triggers as None. Next set Type to Registration. You can then map field data there using [post_INPUT_NAME_HERE] and point your form to the auto actions URL found under the Global tab. If you need to redirect or display a message after they're registered you can configure that under the Output tab.