A warm welcome to you here in the Community, @amdtransport. Let me share a few insights about managing accountant users in QuickBooks Online and make sure he will have access to your company.
Your accountant is prompted to sign up when he accepts your invitation because the system detected that he doesn't have a QuickBooks Online for Accountants subscription yet. I'd suggest you let him sign up to this link: https://quickbooks.intuit.com/uk/accountants/online/. Signing up is free and allows him access to your QBO company at no cost to you.
For future reference, you can also read through our help article: Resend an invitation email. It guides you in the right direction in case the invitation email you sent can’t be found or has expired.
If you have any questions, please don't hesitate to post some more. Cheers!
