How do I add or remove an ad-hoc approver on an Invoice?

Provided you have permissions to add approvers to Invoices, you can add ad-hoc approvers to an invoice via the Approvals section of an Invoice.

Once you have selected an ad-hoc approver to add, you will be prompted to confirm the addition of that approver.

After you have added an ad-hoc approver, you will also be able to remove that ad-hoc approver by pressing the X button after their name. 

Any ad-hoc approvers added on a specific invoice can be removed, but approvers added automatically via Invoice Approvals Settings cannot be removed from specific invoices. Approvers added automatically via your Invoice Approval Settings will have a lock displayed instead of an X, showing you that the approver cannot be removed.