For domain owner,you will receive the access details to log into your domain access (Postmaster login details) and can start create all user account.Guideline as below:
1) Please log into the postmaster account as link per given:
(Mail server name been email by our Support)
2) Enter the details of postmaster access as per email from our Support
3) Following the number on below picture,please navigate to Setting (the gear symbol) -> Domain Settings -> Users -> Click NEW
4) Once appear the User interface as below,please fill up on 'USER' tab only.Please fill in Username,Password and Display Name.The make sure to click 'SAVE'
*NOTE: Smartermail currently require passwords to be a minimum of EIGHT  characters in length with at least ONE UPPERCASE LETTER, ONE LOWERCASE LETTER and 1 NUMBER in the password.Every password will valid for 12 months
5) Kindly repeat the steps for another user of your domain.