How can I correctly control number of users

What I'm trying to do here is control the number of user sessions logged into the application.

So I need Membership.GetNumberOfUsersOnline() to be one less when a user clicks on Log Out.

In my login page_load I have the following:

        Dim intUserCount As Integer
        intUserCount = Membership.GetNumberOfUsersOnline()

        If intUserCount > 5 Then
            Me.Login1.Visible = False
            Me.lblMessage.Text = "You have exceeded your user count."
        End If

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