Extranet Collaboration Manager for SharePoint 2013 (ExCM) allows you to create users in three ways:
1. Manually
2. Invitation Registration (via e-mail)
3. Anonymous Registration (potential users visit the Extranet site and click “Sign Up”)
The most common of these is via invitation. When using invitations, there is most likely an existing business relationship with the invitee and the general thinking is that “if you’re invited, you’re approved.” However, there may be certain scenarios where you want to add an approval step for invited users as well.
In addition, there may be instances where you need to implement Anonymous Registrations for user creation. For example, suppose you didn’t know ALL of the email addresses for potential site users. By using Anonymous Registrations, a user can visit the site, fill out the registration from, and create a new account herself. The downside to this is that since your
SharePoint Extranet site is available worldwide to anyone with an Internet connection, there is a possibility of invalid or unnecessary account creation.
By turning on approval in ExCM, ALL registrations (both invitation and anonymous) are required to be manually reviewed by someone and either approved or rejected…thus adding another layer of security to your site. However, this could become a very tedious task for approvers since they would have to manually check for new accounts periodically and determine their approval status. By creating a basic workflow, you can email approvers when a new registration occurs and let them know it’s ready to be reviewed.
In 2013 you will first need to activate workflows. Go to Site Settings > Site Collection features and activate "Workflows":