When you first sign up for your Hushmail Business account, you will receive a setup email at the alternate contact email address you provided. In that setup email there, is a link to create additional user accounts under Step 1 : Create Additional email addresses.
You can also create a new user account by logging into our webmail interface with the administrator account for your domain.
Once you sign in, please click on Preferences, then select the Domain tab. There will be a link to add user accounts there.