'Override the save method to save the first and last name to the user
field.'

def save(self):

    new_user = super(SignupFormExtra, self).save()
    new_user.first_name = self.cleaned_data[u'first_name']
    new_user.last_name = self.cleaned_data[u'last_name']
    new_user.save()
    return new_user
