Tracking users who have logged in

The easiest way would be to add a static counter that would increment on Session_Start and decrement on Session_End. You'll need to add a variable (like Session["IsActive"] = true;) to the session to allow Session_end to fire properly.

If you want to track by-user you will need to add a static IEnumerable (or a class I suppose, if you prefer) that is similarly manipulated by the Session Start and end events, but using either names or your database keys.

public class MvcApplication : HttpApplication
{
    public static List<int> UsersOnline{ get; set; } // or string, or custom class

    protected void ApplicationStart()
    {
        UsersOnline = new List<int>();
    }

    protected void Session_Start(object sender, EventArgs e)
    {
        UsersOnline.Add(/* Hook to DB or Directory here */);
    }

    protected void Session_Start(object sender, EventArgs e)
    {
        UsersOnline.Remove(/* Hook to DB or Directory here */);
    }
    ...
}
</int></int>