code: def save(self, *args, **kwargs):
          
          try:
              user = get_user_model().objects.get(
                  email__iexact=self.cleaned_data["email"]
              )
          except get_user_model().MultipleObjectsReturned:
              raise forms.ValidationError(
                  _("This email address has been used multiple times.")
              )
          except get_user_model().DoesNotExist:
              user = invitation_backend().invite_by_email(
                  self.cleaned_data["email"],
                  **{
                      "domain": get_current_site(self.request),
                      "organization": self.organization,
                      "sender": self.request.user,
                  }
              )
          # Send a notification email to this user to inform them that they
          # have been added to a new organization.
          invitation_backend().send_notification(
              user,
              **{
                  "domain": get_current_site(self.request),
                  "organization": self.organization,
                  "sender": self.request.user,
              }
          )
          return OrganizationUser.objects.create(
              user=user,
              organization=self.organization,
              is_admin=self.cleaned_data["is_admin"],
          )
docs: The save method should create a new OrganizationUser linking the User
          matching the provided email address. If not matching User is found it
          should kick off the registration process. It needs to create a User in
          order to link it to the Organization.