## Is your feature request related to a problem?

Currently if you login with SAML and you don't have an account on the PostHog instance one of two things happen: a) either a new account is created **if the email matches the domain whitelist** or b) you get turned down at the door. Yet after some thought, the point of SAML is that you manage user provisioning in a separate centralized system, so we shouldn't have the extra domain check. 

## Describe the solution you'd like

If a user logins through SAML and their account doesn't exist, create it automatically, regardless of whether it matches the domain whitelist.


## Describe alternatives you've considered

Keeping as is.

## Additional context

There's a complicating factor which is that SAML login is instance-based yet user signup is organization-based, so there's the edge case of an instance having multiple orgs. Should the user then be signed up to all organizations?

#### *Thank you* for your feature request – we love each and every one!

posthog/api/signup.py
invite.target_email,
                        validated_data.pop("password"),
                        validated_data.pop("first_name"),
                        **validated_data,
                    )
                except IntegrityError:
                    raise serializers.ValidationError(
                        f"There already exists an account with email address {invite.target_email}. Please log in instead."
                    )
            try:
                invite.use(user)
            except ValueError as e:
                raise serializers.ValidationError(str(e))
        if is_new_user:
            login(
                self.context["request"], user, backend="django.contrib.auth.backends.ModelBackend",
            )
            report_user_signed_up(
                user.distinct_id,
                is_instance_first_user=False,
                is_organization_first_user=False,
                new_onboarding_enabled=(not invite.organization.setup_section_2_completed),
                backend_processor="OrganizationInviteSignupSerializer",
            )
        else:
            report_user_joined_organization(organization=invite.organization, current_user=user)
        
        user_identify.identify_task.delay(user_id=user.id)
        return user
class InviteSignupViewset(generics.CreateAPIView):