Can you please provide additional information about what you're referring to when the email doesn't work? Did you receive any errors while inviting the user? I'd appreciate any further details so I can present accurate information. While waiting for your response, I'll share different scenarios about inviting a user and steps to resolve this.
If you mean that the email address of the invited user didn't work while you're setting up the invitation, I suggest making sure that the email used doesn't have extra space. If the issue persists, I suggest logging your QuickBooks account using a private browser. This is to rule out the possibility of a webpage issue, and private browsing doesn't store local files or cache. This might be also the reason that you encountered a problem with the email used.
Use these keyboard shortcuts based on the browser you're using.
For Firefox, press Ctrl + Shift + P.
For Chrome, press Ctrl + Shift + N.
For Safari, hold down Command + Shift + N.
If it works in incognito, I recommend clearing the cache of your regular browser. This removes the history or log of sites so you can start with a clean slate. You can check this article to see the steps: Clear cache and cookies to fix issues when using QuickBooks Online.
If you're referring that the user didn't receive their invitation or their invitation is missing, there are reasons your invited users not receiving their email invitations. Please check below:
The invitation might be in their junk or spam folder.
The company’s email address is incorrectly formatted in Company settings.
The recipient’s mail servers might be blocking the email.
The invitation may have already expired. Email invites expire after 30 days from when it was sent.