Let me add a few input, mareti_cpsi.

Your user has an account in our system. Deleting and re-adding them will still route to the sign in page.

After trying @PreciousB's suggestion, let your user contact our QuickBooks Online Customer Care. A phone agent can update their information and help them reset the password.

If you have other concerns, you can always come back anytime. We'd be happy to help you.