Hi there, @Lottie--D.
You can invite them again by changing the email address of your employee in the Employee Center. Then, try deleting and recreating the invite to see if you still encounter issues. If so, you can try adding your employee with a different email address.
To know the detailed process, you can check out this article: Invite your employees to QuickBooks Workforce to see pay stubs, W-2s and more.
I also added this article so you can notify your employees of new pay stubs whenever they're already available to view online: Email pay stubs from QuickBooks Desktop.
Let me know if you have more questions about sending an invitation to your employees. I'll be right here to help you anytime.