@Mentioning and Sharing a proof requires the new user mentioned to submit an approval status, which may delay the progression of the review. If the proof workflow is sequential, the @mentioned user will be added to the end of the workflow and will not receive an email notification or have access to the proof until it is their turn in the review. If the proof workflow is tiered, the @mentioned user will be invited at the end of the current tier.
The invited reviewer is considered optional until they open the review. If they open the review, they are then considered a required reviewer. If they do not submit their review before a tier deadline is met, this can delay a functioning workflow.