## 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
serializer_class = InviteSignupSerializer
    permission_classes = (permissions.AllowAny,)
    def get(self, request, *args, **kwargs):
        """
        Pre-validates an invite code.
        """
        invite_id = kwargs.get("invite_id")
        if not invite_id:
            raise exceptions.ValidationError("Please provide an invite ID to continue.")
        try:
            invite: OrganizationInvite = OrganizationInvite.objects.get(id=invite_id)
        except (OrganizationInvite.DoesNotExist, ValidationError):
            raise serializers.ValidationError("The provided invite ID is not valid.")
        user = request.user if request.user.is_authenticated else None
        invite.validate(user=user)
        return response.Response(
            {
                "id": str(invite.id),
                "target_email": mask_email_address(invite.target_email),
                "first_name": invite.first_name,
                "organization_name": invite.organization.name,
            }
        )
class SocialSignupSerializer(serializers.Serializer):