In Proto.io creating reviewers is part of the Share Project process.
Once you create a Share link for your project, you go to the SHARE WITH tab and select the people who will have access. You can create a reviewer by simply entering their email address in the 'Add reviewer’ field. An invitation will be sent for them to sign up.
The 'Notify by email' option will send an email to everyone who has access to the particular Share link, sending them the actual Share link for reviewing the project. Learn more.
All reviewers you create while sharing will later appear under USERS in your Dashboard.