Step 1: Sign in to Google Domains
Step 2: Select the applicable domain.
Step 3: Click "Registration settings" on the left-hand side:
Step 4: Scroll down to "Users" and click "Add user" in blue.
Step 5: Enter "email@example.com" in the Email address field and provide Full permissions. Click the blue "Add" button in the bottom-right.
That's it, we'll get an email notification on our end that you've added us as an authorized user to your domain.
If you happen to run into any issues or have any questions at all, please be sure to let us know! We're always very happy to help.