Hi John! We use proofs regularly and have our external customers approve the work we are doing for them. For those external customers we simply add their email address to the proof contacts and they receive it to review and approve.
If you are not able to do that, check the bottom area of your proofing window and ensure that the “require login” option is not checked.
If you are on NWE you cannot add an email to a Proof but that update is coming later this week which will allow you to add email addresses again.