ASP.NET 5 Windows Authentication - search Active Directory users

We are using ASP.NET 5 windows authentication in Visual Studio 2015 web application project. We notice that the default MVC template displays the logged in user name as Domain\username using User.Identity.Name. We want to implement a user management functionality by searching a user in the AD domain and then assigning a role to that used. How do we search a user by name in AD. For example, we want to search for John Smith to make sure he exists in the domain and then assign a role to that user, how would we search for this user in AD using AP.NET windows authentication? Thanks..Nam