If you want to privately share with your team (including Designers and/or Reviewers) for review, then you can choose this option. This means that the Share link is only accessible by the people you give access to, upon signing in with their Proto.io account credentials.
You enter their email addresses and add them to the list of people who have access. They are instantly given access to the particular share link. If they click on the Share link and already have a Proto.io account, they can sign in and preview it, otherwise, they will be guided through the Reviewer signup process first. (You can also see How do I add reviewers?)
You can choose to notify them if you want. They will get a standard email notifying them about reviewing this shared prototype, or you can choose to edit your own message.
The person who receives this share link can view it in a browser window, or in the Proto.io app.