Add User in free mail account

Hi there! Just created a free mail account for my business. Would like to add my partner as a user. I followed instructions in the setup process and first tried it with his primary gmail address. He got a notification email and everything but when he went to the site he was prompted to create an @zoho.com email address instead of one at our domain. I tried it by doing the other option of putting all details in for him and when he tries to log in it says his password is bad or that the user doesn't exist. Wondering if I can get some specifics on how to get the user I want to add onto the site?