Your customers cannot use their Zoho credentials to log in to the customer portal.
When you send a portal invite to your customer and they accept it, they will be requested to create a new password, which they can then use to log in to the portal. Their Zoho credentials are irrelevant in this process. Even customers without a Zoho account can log in to your portal using their email ID and the password they created.
Books
VAT compliant accounting
software for small
businesses.