void Session_OnStart(...)
{
 // when session starts. code goes here ... eg
 int counter = int.Parse(Session["onlineUsers"].ToString());
 counter++;
 Session["onlineUsers"] = counter;
}

void Session_OnEnd(...)
{
 // when a session ends
 int counter = int.Parse(Session["onlineUsers"].ToString());
 counter--;
 Session["onlineUsers"] = counter;
}