Using Community Auth on Existing Project

Codeigniter doesn't come with native login system, so the script you purchased is already using a third party package/plugin.

Look for login script code in the following locations : Models, Libraries. The controller is where you control everything, so you can have a look at that later, first you need to determine which files are being used as the core of login functionalities. Once you have determined, you can look at the code and understand how they are handling user groups/ previlages etc.