CodeIgniter authentication + user privileges
There Are many libraries that already handle Authentication within Codeigniter, but the one I would recommend is Ion_Auth. It handles user permissions (groups) very well and I've actually done a detailed writeup outlining a good way to handle this situation with Ion_Auth.
I suggest FlexiAuth library which is a re-modified version of ion Auth, and has lot of features already built in, simply out of the box.
I've been developing a role based authentication system for Codeigniter called WolfAuth, here's the repository. It's still early days, but it sort of works. You can add roles, etc and then restrict functions and classes to particular user roles and usernames.