Add a member of the enterprise to one of its organizations.

Adds them at once, with no invitation to accept, and emails them to say so. Only people in the enterprise's directory can be added: anyone else, a guest or a removed member included, is a 404 (enterprise_member_not_found), and someone already in the organization a 409 (enterprise_organization_member_exists). The owner role is not grantable here (403), and a role the organization's plan does not offer is a 403 (shorebird_plan_insufficient_tier). Admins only.