set a value for the checkbox:
<input id="send_cc" name="send_cc" type="checkbox" value="1" />
then write an if statement:
If Request.Form("send_cc") = "1" Then
set WA_MailObject = WAUE_AddCC(WA_MailObject,"" & cStr((rsUsers.Fields.Item("UserEmail").Value)) & "")
End If