how to create a login / registration system

Have a look at Ion Auth, an existing login/registration system built for CI.
You will learn that it involves:
- a controller that collects data for displaying it on forms and pages, and also process the data coming back from forms.
- a library with common functions e.g. to check if a user is member of a certain group
- a model with database functions (get, save, update, delete)
- a number of views, e.g. a login form
- a number of configuration files, e.g. to make the system multilingual
- a table in your database that holds the registered users' data
- another table that holds the group names
- another table that holds the relationships between users and groups
Make the library globally available by putting it in the autoload.php config file.
Also, make use of the session driver to make all your pages remember the logged in user's id.

Direct your user to a login form.
After submitting the form you check if the user's name (or email) and password are correct. If so, store the user's id in the session data.
If the user opens a url on your site which has limited access (only for a certain group), you check if the user's id is in this group. If not, you deny access to the page.