I've had this problem since October 2019.  I gave up eventually, using TSheets instead for the two impacted employees.

Last week we hired a new employee, I was able to add her as a Time Entry user during the initial "Add Employee" setup.  Thinking that this problem has been resolved, I tried to add one of those employees I couldn't add last year.  First with the email address on file for the employee - failed as last year.  Then I tried with a new email address, failed same as last year.

I cannot believe a large commercial company like Intuit would have this sort of problem, impacting so many users, for almost two years so far.  What is the solution, besides using TSheets?

Ten months ago I had this reply from Jen_D (QuickBooks Team):

"I completely understand where you're coming from, and can relate to the inconvenience this has caused you. 

This problem is something our engineers are still working on up to this moment. They found out that inviting a user that is already added in the program generates the error message. We don't have the turn around time on when they will release a fix for this hitch.

As an alternative, you may invite users using an alternate email or continue entering the time data manually.

If you're not yet added to the notification loop, I'd recommend getting in touch with our Support Team so they can add you to the investigation ticket number INV-28913. Here's how to contact us..."