Zoho CRM Portals - allow access per account

Zoho CRM Portals - allow access per account

Hello all,

I am trying to set up a portal for our customer but I seem to be hitting an obstacle and I am not sure if it is my problem or a limitation in the software.

So basically the way I understand the portal
Contact Email > Each Record or Related Record that is linked to that email > can be shown

But what I want to achieve is to give access not to Contacts, but to Accounts.
Lets say we have an Account who is a customer of ours. There are two people in that account that order with us.
Currently we can create
Contact A > Has access to Sales Invoices A
Contact B > Has access to Sales Invoices B

What I want to create is 
Contact A and B and C > have access to Sales Invoices A B C
My wish is that for example
Contact A > sees only Sales Invoices A
But the owner (CEO of my customer) can see all invoices, cases, etc. created for his company

Thank you for your time!