Once your web services is enabled, you will have the option to allow users to request a New Account from the login page.
First, setup your login page.
Create New Account Customization
You can customize your create new account form in the Settings module under the Web Services > General > Login tab. Here you can add additional fields to require, including SSN. You can also require that this field be unique, which can mitigate manual confirmations for duplicate accounts.
Below is the flow for Create New Account.
Create New Account Flow
The Create New Account process is initiated by the person who is requesting the account, when they click Create Account on the login page.
Test It Out
I encourage you to use a test email/record to test this process and follow along with the flowchart above.