trying to create a new user with full access

Hello there, @deirdre6. You can have your employee check the invitation email in their junk or spam folder. Your customer might have enabled a setting that would automatically categorise an unknown email to spam. Or the recipient’s mail servers might be blocking the email from landing in their inbox.
 
If this isn't the case, your company's email address could have been formatted incorrectly in the Company tab under Account and Settings. You can read this article for more details: What to do if invited users did not receive your email invitation in QuickBooks Online.
 
You can always return here if you have additional questions.