In Odoo V16 you prevent website visitors from creating their own account a little differently than in V15. To make sure accounts are created on invitation only, follow this step.
Set accounts to invitation only
Go to the Website app -> Configuration -> Settings -> Shop - Checkout Process and select Disabled (buy as guest).
From now on a visitor can no longer create an account during checkout. You grant portal access yourself, by sending a user an invitation.
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