You can create new users in Proto.io from your Dashboard (owners and admins can do this). You will need to give their name, a valid email address and select an admin or designer role.
Reviewers are created while sharing your project and are given access to specific Share links (see Sharing with reviewers). You can create a reviewer by simply entering their email address in the 'Add reviewer' field (see image below). An invitation will be sent for them to sign up.
When creating designers, you will have to assign them (i.e., give them access to) to the project(s) you want.
Newly created users receive a notification that guides them through the signup process.