The best thing to do is to follow the securityAssist tutorials. If you combine powerstore and powerCMS then all of the registration and login information will already be done for you, but Security Assist documentation will help you understand how it all works.
You can take advantage of the SecurityAssist server behaviors for restricting page access and setting session variables to define user groups. It is all covered in the documentation... maybe go through the documentation and tutorials on securityAssist and then you will be able to recognize what is already working in powerstore and take advantage of it for your needs.