At the moment the designed solution is that when a Workfront user who has access to a project to which a proof has been added and who the proof has not been explicitly shared before opens the proof from within Workfront, they are added to the proof immediately afterwards.
This is a poorly designed system setting. Just because a user clicks on a proof to open it does not mean that they should be added to the proofing process. Users should decide who and when someone is added to a proof. If a user clicks on a proof but is not part of the proofing process, they should be able to view it without having to become a reviewer. This is especially annoying for system admins and project managers who are just trying to see where something is at in a project and aren't actually involved in the review process.