Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
' Fires when the session is started
Application("UserCount") = Application("UserCount") + 1
End Sub

Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
' Fires when the session ends 
Application("UserCount") = Application("UserCount") - 1
End Sub

To Display UserCount, add a Label Control to the WebForm, name it lblUserCount

Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
'Put user code to initialize the page here
Me.lblUserCount.Text = "User(s) online " & Application("UserCount")
End Sub