Use of methods in internal classes from referenced dlls

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C#

DLL

webservice

I have a pretty big web app which consists of a few different projects. I have to add some extra functionality (account validations, searching products etc). I was provided with two .dll files let's say Services.dll and Interfaces.dll, which look like that:

//SERVICES.DLL
using Interfaces;

namespace Services
{
  internal class Users : IUser
  {
    public Users GetUserByName (string Name)
    {
      //Implementation
    }
  }
}

//INTERFACES.DLL
namespace Interfaces
{
  public interface IUser
  {
    Users GetUserByName(string Name);
  }

  public class Users
  {
    public string FirstName {get; set;}
    public string LastName {get; set;}
  }
}

I have two choices for using them. Either create a web service and pass it as service reference in my app or add the actual dlls as references. I don't have decided which would be better yet but I have a question on how will I use the methods of services.dll in my app. I mean that the class is declared as internal so I can't get nor use its methods. Any ideas on how to solve it are welcome.