Have you tried adding a user to your Google Ads account only to get an error saying this email address isn’t an allowed domain? This is an extra security layer by Google and the fix is quite straightforward once you know what to do.
If you’re still on the page to add a new user, navigate to the Security tab at the top as shown in the screenshot here. Then go down to the Allowed domains and add your user’s domain.
Example: if the email address you want to add is bob@example.dom, then the domain to add is example.com.
Now you can go back to the Users tab and add your new user.
If you also need help adding a new user to GTM, here's how.