There is a Reviewing Proofs pdf that seems to be the go-to for Proof review/approval functionality training. There are a number of outdated links in the document, and it also shows viewing from Home, which an external user can't see.
A concise (not 20pg) document which simply explains how to review static/motion etc. assets, comment and approve Proofs from an email link for an external user only is crucial.
I have seen discussion on this, and it seems people have had to create their own documentation on this repeatedly, myself included. Thanks