Here are a couple links that might help. I think it depends on if you
created the proof and the profile of the person you're trying to
mention, if they're inside or outside of your company.
https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FReview_and_approve_work%2FProofing%2FReviewing_Proofs_within_Workfront%2FComment_on_a_proof%2Ftag-users-to-share-proof.htm&_LANG=enus
https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experienc...