Hi,

By default, when a new portal user registers on a website in a multi-company setup, Odoo only assigns them to the website’s company. If you want them to have access to two companies automatically, you need customization: either create an automated server action on res.users to add the second company when the user is created, or build a small custom module that overrides the signup process. Out of the box, Odoo won’t grant access to multiple companies on registration.

References:

* https://www.odoo.com/documentation/16.0/applications/websites/website/configuration/multi_website.html

* https://www.odoo.com/sl_SI/forum/pomoc-1/multi-company-multi-website-multiple-portal-accounts-246573

* https://www.odoo.com/sl_SI/forum/pomoc-1/odoo-13-how-to-ensure-that-portal-user-is-redirected-to-the-correct-website-in-a-multi-website-environment-171402

Hope it helps