Add Active Directory Authentication to existing CodeIgniter Project

I have an existing Codeigniter project that has two main views; one for 'regular' users, and another for 'admin' users. I need an the project to authenticate with Active Directory. If a user in "Group A" authenticates, they will get access to the admin view, if a user in Group B authenticates, they will get access to the regular user view. You must have your own environment to code in (access to a Windows Server environment).