We haven't received any reports with the same issue as this, hhimlin.

I'm here to ensure your employees can submit their expense claims.

Let's ensure that their email addresses are correct. These addresses are case sensitive. Thus, any space or incorrect letter case can be the reason why they didn't received your invitation. 

 If the details are correct, let them review their Spam inboxes. It might be the IP addresses are marked as spam by some of their email providers. You can ask them to white-list the following QuickBooks Online email server hostnames and IP addresses so their invitation won't get recognized as spam.

For additional reference, I've attached this article that you can use to enter transactions, like how you can pay your employee a reimbursement: Reimburse an employee.

Let me know in the comment section if you need help with the reimbursement process.