To add a new user to Google Postmaster Tools, follow these steps:

1. Log in to https://postmaster.google.com/ and hover over the domain to which you would like to add a new user and click the 3 dots to the right of it. A context menu will pop up.

2. Click on the 'Manage Users' link in the context menu.

3. Click on the big red '+' button located at the bottom right of the screen.

4. In the popover that appears, enter the email address of the new user (e.g., hello@sendx.io) and click 'Next'.

5. That’s it! The new user will receive an invitation to access the domain's data in Google Postmaster Tools.