How do I add multiple users?

How do I add multiple users?

Hi, sorry I am new to Zoho mail. We're a small company of two people, and we have three email addresses. 

IE. A main sales@ email, and our two personal emails.

I want my cofounder to have access to all of the email inboxes at the same time, and I want access to his email at the same time, IE. Multiple viewable inboxes.

I cannot for the life of me figure this out on Zoho Mail.

Just to reiterate because I know I've probably described this poorly; we would each like to have access to all of the Zoho email addresses from our accounts, ideally in the same inbox.