This module will give your users the abaility to register and login in one step>
user will add his email and press enter and all DONE!!
A new user account will made for him with auto generated password.
He will logged in to your site and get an email with his login details.
You can integrate community builder with this module and add user automatic to CB.
You can ask user to enter a password and not to generate it automatic.
you can set some html code before and after the email field.
You can disable auto login function.