Where Store Admin Information?

I recommend an authentication library for CI, like Ion Auth from Ben Edmunds. It comes with tables for users, groups and users_groups. And a lot of powerful functions in it's library.
Example:

PHP Code:

$this->ion_auth->is_admin(); 


This will return a boolean (true or false) to check if the current user is a member of the group "admin".

Also, you can check if a user is member of any other group, like this:

PHP Code:

$employee = $this->ion_auth->in_group('employee');