Sitefinity User VS Portal User
A Sitefinity user for a portal site can be a System Admin who has back-end access to Portal Site or a Developer who also has access the Portal Site Back-End to develop Page Templates, Forms & Pages etc.
An end user is also be a Sitefinity User but without access to the site backend.
A Sitefinity end user can only view general content on the site that is publicly available or they can view the contents which are secured to specific group of users based on their role.
A portal user is similar to Sitefinity end user but he can create/update/delete his related CRM records through a portal site. A portal user has a related contact record in CRM and a portal user record along with a portal user role record in CRM. These fields are connected to the Portal users' Sitefinity profile with a set of hidden fields on their profile page.