PostgreSQL offers robust features for managing data in multi-tenant applications, but implementing fine-grained permissions requires careful planning. This guide outlines creating a permission system for SaaS applications with multiple organizations, focusing on tenant isolation, role-based access control, and scalable architecture using PostgreSQL's built-in capabilities such as roles and Row-Level Security (RLS). The process includes setting up roles and permissions, implementing tenant isolation through RLS, designing a data model supporting multi-tenancy with row-based tenancy, and integrating role-based permissions. It also covers integrating this permission system into a Node.js application using session variables to manage user and tenant context. The guide emphasizes security best practices, potential pitfalls, and the benefits of using a dedicated authorization solution like Permit.io for more advanced features such as attribute-based access control and audit logging.