To create an user in Joget Workflow user directory using form created in Form Builder, you can set Form Store Binder of a form to "Directory User Form Binder". 
Design the form to capture user information as below: 
Your form must contain the field with the following field id:
1. username
2. firstName
3. lastname
4. email
5. password (will encrypt by md5)
6. active (either '0' or '1' value, '0' mean inactive and '1' mean active)
7. timezone (In GMT format, '8' is the time zone for 'Kuala Lumpur' and '-6' is 'Mexico City'. This field is optional.)
8. userRoles (either 'ROLE_USER', 'ROLE_ADMIN', or 'ROLE_USER;ROLE_ADMIN')