When changing the status of one or more screens, a banner will appear at the bottom of the screen asking if you'd like to notify collaborators. That notification can only be sent out to collaborators/members of the prototype and cannot be sent to external users at this time.
If an external reviewer (someone that is neither a Collaborator nor a member of the Enterprise organization) needs to review the screen, they can do so via share link but they won't be able to change the screen status. Instead, they'll need to add a comment with their approval or request to change status, and then a member of the prototype will need to change the status.
We’d generally recommend that Enterprise customers add reviewers formally as Reviewers in their Enterprise organization so that the reviewer can set screen statuses and view prototypes without needing to be given a share link every time.