Whenever, you enable portal access, an invitation mail will be triggered automatically from Zoho Books. Your customers need to accept this invitation in order to access the portal.
You can also re-invite your contact to the portal. To do this please follow the below steps:
An invitation mail with a link to set up a password will be sent to the contact person. On clicking the invite link, Zoho Books will open up another page where your client can create a password to login to your portal. Once they login, they can view/pay/add comments to the invoice.