How much does the Multi-user add-on cost?

The packages within the Email plan do not include any additional user account. The PRO plan offers three and the Max  five account users in a package (including the account owner).

On the top of that, you can add the Multi-user add-on to your plan for $20 per month, which includes up to five users. To purchase the add-on, go to My account>>Add-ons and click Buy Now beside Multi-user. The charge for the add-on will continue every 30 days until you disable the auto-renewal. You can buy more users for an additional $5 each per month (up to 25 limit).