You may be asked to add a reviewer to an older version of a proof if they weren’t included on the proof at the time.
As the proof owner, you can either:
Add them as a reviewer into the workflow in the proof’s info pane.
Note: Because the proof has a new version they won’t receive an invitation to review the proof since the red pen is no longer available.
Add them as an owner (they will have the same priviledges as you do)
To add an owner, follow the steps in this help article.