Через сессию не получится!!
Нада через Aplication:

protected void Application_Start(Object sender, EventArgs e)
{
Application["visits"] = 0;
Application["active"] = 0;
ArrayList ar=new ArrayList();
Application["Users"]=ar;

}

protected void Session_End(Object sender, EventArgs e)
{
Session.Abandon();
Application.Lock();
int n=(int)Application["active"] - 1;
Application["active"] =n<0?0:n;
ArrayList ar=(ArrayList)Application["Users"];
if(ar.Count>0)
{
int i=ar.BinarySearch((string)Session["NameUser"]);
if(i>=0)
ar.RemoveAt(i);
}
Application["Users"]=ar;
Application.UnLock();
}

protected void Application_End(Object sender, EventArgs e)
{
Application.Lock();
Application["visits"] = 0;
Application["active"] = 0;
ArrayList ar=new ArrayList();
Application["Users"]=ar;
Application.UnLock();
}
и на первой странице при регистрации
вот это:
Session["NameUser"]=mName;

Application.Lock();
Application["visits"] = (int)Application["visits"] + 1;
int intTotalVisitors = (int)Application["visits"];
Application.UnLock();
Session["VisitorID"] = intTotalVisitors;
Application.Lock();
Application["active"] = (int)Application["active"] + 1;

ArrayList ar=(ArrayList)Application["Users"];
if(ar.Count==0 || ar.BinarySearch(mName)==-1)
ar.Add(mName);
Application["Users"]=ar;

Application.UnLock();
Теперь у тебя в Application["Users"] все пользователи единственное, то что
Session_End(... вызывается Web server-ом по истечении таймаута