Zoho Business/Email user account confusion

I'm just setting up my first company using Zoho Business so I hope this isn't a stupid question, but user accounts are confusing to me.

We had already registered our own domain, so after making myself a Zoho Business trial account I set up our domain and validated it. That worked fine.

Then I went into the Control Panel/Users and added a user account.

The first problem is that the instructions told me the user would receive an e-mail and had to respond to that e-mail to activate their account. But I'm just setting up their e-mail account so how can they respond if they can't log in? It seems that those instructions are incomplete, or inaccurate because I simply tried to log in as that new user and it worked without any e-mail activation.

The other confusing part is when I looked around as the new user. As the new user I clicked on "Settings" at the top of the page, then "Mail Settings" expecting to configure my new e-mail account. But in the tab at the top of the page it says "username@zoho.com" which is not what our users will expect to see - they expect to see their e-mail address in our domain.

It gets more confusing because the "Settings" page also has a link to "My Email IDs" which I can't figure out at all. It lists the proper e-mail address (in our domain) that I assigned to that user, but it also has an "Add" button with no other information. I don't want my employees adding any other arbitrary e-mail addresses to our account, if that's what this is used for.

Can someone explain, or point me at some documentation to help me understand this?

Thanks.