A proof owner has full management of the proof (managing the workflow, delete, archive, adding other users etc). Adding and removing proof owners is done in the proof’s info pane.
To add/remove a proof owner
Open the proof’s info pane by clicking the i icon.
Adding an owner
In the owners section, click the plus icon. Then enter their email address and click add.
Removing an owner
In the owners section, hover over the owner’s avatar and select remove.