Challenge making code testable

I'm refactoring legacy code to make it (unit) testable. I have two classes implementing the same interface, as shown below. In one of the classes I have an external dependency towards DirectoryEntry. My suggested way of dealing with that is to create an IDirectoryEntry interface, and pass the directory entry as a parameter to the GetGroups method. The problem is that that would break IMyInterface.

What would a better way to refactor this code?

public interface IMyInterface
{
    List<string> GetGroups();
}

public class MyFirstClass : IMyInterface
{
    public List<string> GetGroups()
    {
        ..
        var directoryEntry = new DirectoryEntry(path);
        ..
        return something;
    }
}

public class MySecondClass : IMyInterface
{
    public List<string> GetGroups()
    {
        return somethingElse;
    }
}

Regards, Frank