When you first signed up for your Hushmail Business account we would have sent you a set-up email to an alternate contact email address. In that set-up email there is a link to create accounts under Step 1 : Create Additional email addresses.
You can also create a user account by logging into your admin Hushmail account in webmail at https://www.hushmail.com.
Once you log in click on Preferences then the Domain tab. There will be a link to add user accounts there.