Checkout this documentation.
A brief use case:
A company, who has more than 2500 clients, sells products only to customers through a private website. Invitations are sent to them to register to the website. In Odoo they need to be granted portal access to make that registration.
In the following documentation, you'll find how to import a list of portal users and how to masively grant them portal access.
Documentation:https://docs.google.com/document/d/1Gck7C-iYdXyIW_1U4IhDYGeBZ7dI0aG3OYsUdkS-ANg/edit
