[Answered ]-Django registration redux registration only with email (no password)

User model

First you need to create custom User manager and model.
Documentation can be found here https://docs.djangoproject.com/en/1.9/topics/auth/customizing/#a-full-example

from django.contrib.auth.models import AbstractBaseUser, PermissionsMixin, BaseUserManager

class EmailUserManager(BaseUserManager):
    def create_user(self, email, password=None):
        if not email:
            raise ValueError('Users must have an email address')

        user = self.model(email=self.normalize_email(email),)
        user.set_password(password)
        user.save(using=self._db)
        return user

    def create_superuser(self, email, password):
        user = self.create_user(email, password=password,)
        user.is_admin = True
        user.is_superuser = True
        user.save(using=self._db)
        return user


class EmailUser(AbstractBaseUser, PermissionsMixin):
    email = EmailField(
        verbose_name='email',
        max_length=255,
        unique=True,
    )
    is_active = models.BooleanField(default=True)
    is_admin = models.BooleanField(default=False)
    date_joined = models.DateTimeField(default=timezone.now)

    objects = EmailUserManager()

    USERNAME_FIELD = 'email'

    def get_full_name(self):
        return self.email

    def get_short_name(self):
        return self.email

    def __str__(self):
        return self.email

    @property
    def is_staff(self):
        return self.is_admin

And register it in your settings.py with this line AUTH_USER_MODEL = 'app.EmailUser'.
Now your EmailUser model can be used with from django.contrib.auth import get_user_model and from django.conf.settings import AUTH_USER_MODEL.

Example usage (foreign key):

from django.db import models
from django.conf.settings import AUTH_USER_MODEL

class Profile(models.Model):
    class Meta:
        verbose_name = 'Profile'
        verbose_name_plural = 'Profiles'

    user = models.OneToOneField(AUTH_USER_MODEL, verbose_name='User',
                                related_name='profile')
    full_name = models.CharField('Full name', max_length=64, blank=False)
    phone = models.CharField('Phone', max_length=64, blank=True)

Registration

When using django-registration-redux two-step registration:

• Create User with None password on registration. This will make User model with unusable password https://docs.djangoproject.com/es/1.9/ref/contrib/auth/#django.contrib.auth.models.User.has_usable_password. It may be necessary to create custom RegistrationForm and RegistrationView to avoid password checking.
• On activation form submit (activation form template with password field) use methods RegistrationManager.activate_user(key) and User.set_password(password)