State design pattern modifying context class collection - How to unit test.

I am currently using nSubstitute and nUnit to do some unit testing. I a newbie when it comes to this stuff so any help would be greatly appreciated. Please have a look at this code:

public class Event
{
    private ISet<Host> hostSet; 
    private IEventState eventState; 

    public Event()
    {
        hostSet = new HashSet<Host>(); 
        eventState = new StartedEvent(); 
    }

    public virtual string Name { get; protected internal set; }

    public virtual bool CanSetName()
    {
        return eventState.CanSetName(); 
    }

    public virtual void SetName(string name)
    {
        eventState.SetName(this, name);
    }

    internal virtual ISet<Host> HostSet
    {
        get
        {
            return hostSet; 
        }
    }

    public virtual ReadOnlyCollection<Host> HostCollection
    {
        get
        {
            return new ReadOnlyCollection<Host>(hostSet.ToList()); 
        }
    }

    public virtual bool CanAddHost()
    {
        return eventState.CanAddHost(); 
    }

    public virtual void AddHost(string firstName, string lastName)
    {
        eventState.AddHost(this, firstName, lastName);
    }

    internal virtual void ChangeState(IEventState eventState)
    {
        this.eventState = eventState; 
    }

}

public class Host
{
    private readonly string firstName; 
    private readonly string lastName;

    public Host(string firstName, string lastname)
    {
        this.firstName = firstName; 
        this.lastName = lastname; 
    }

    public string FirstName
    {
        get
        {
            return firstName; 
        }
    }

    public string LastName
    {
        get
        {
            return lastName; 
        }
    }
}

public interface IEventState
{
    bool CanSetName(); 
    void SetName(Event evnt, string Name); 
    bool CanAddHost(); 
    void AddHost(Event evnt, string firstName, string lastName); 
}

internal class StartedEvent : IEventState
{

    public bool CanSetName()
    {
        return true; 
    }

    public void SetName(Event evnt, string name)
    {
        evnt.Name = name; 
    }

    public bool CanAddHost()
    {
        return true; 
    }

    public void AddHost(Event evnt, string firstName, string lastName)
    {
        evnt.HostSet.Add(new Host(firstName, lastname)); 
    }
}

[TestFixture]
public class StartedEventState
{
    [Test]
    public void CanSetName_Always_ReturnsTrue() 
    {
        IEventState eventState = new StartedEvent();
        bool result = eventState.CanSetName();
        Assert.True(result);
    }

    [Test]
    public void SetName_WhenCalled_ChangesTheEventName() 
    {
        Event evnt = Substitute.For<Event>();
        IEventState eventState = new StartedEvent();
        eventState.SetName(evnt, "Test Event");
        evnt.Received().Name = "Test Event";
    }

    [Test]
    public void CanAddHost_Always_ReturnsTrue() 
    {
        IEventState eventState = new StartedEvent();

        bool result = eventState.CanAddHost();

        Assert.True(result);
    }

    [Test]
    public void AddHost_WhenCalled_AddsTheHostToTheEvent()
    {
        //No idea how to test this? 
    }
}

[TestFixture]
public class EventTests
{
    [Test]
    public void CanSetName_WhenCalled_DelegatesToItsCurrentState()
    {
        Event evnt = new Event();
        var eventState = Substitute.For<IEventState>();
        evnt.ChangeState(eventState);
        evnt.CanSetName();
        evnt.Received().CanSetName();
    }
}

The tests that are currently written all pass. However I have no idea how to perform a test to ensure that the StartedEvent.AddHost method is correctly calling the context class and adding it to the set. I have tried a few different things but have failed miserably.

1) Can someone point me in the right direction as to how I go about testing this method. 2) It would be good if someone could confirm my existing tests are OK.