UsersActor.cs

public class UsersActor : ReceiveActor
{
    private Dictionary<string, IActorRef> _users;

    public UsersActor()
    {
        _users = new Dictionary<string, IActorRef>();

        Receive<StartedReadingMessage>(message => ReceivedStartMessage(message));
        Receive<StopedReadingMessage>(message => ReceivedStopMessage(message));
    }

    private void ReceivedStartMessage(StartedReadingMessage message)
    {
        IActorRef userActor;

        if(!_users.TryGetValue(message.User, out userActor))
        {
            userActor = Context.ActorOf(Props.Create(typeof(UserActor)), message.User);
            _users.Add(message.User, userActor);
        }

        userActor.Tell(message);
    }

    private void ReceivedStopMessage(StopedReadingMessage message)
    {
        IActorRef userActor;

        if (!_users.TryGetValue(message.User, out userActor))
        {
            throw new InvalidOperationException("User doesn't exists!");
        }

        userActor.Tell(message);
    }
}