Well, I assume you already know how to make the radio thing. Ok, make the radio as normal. Now, to have the radio channel hidden, you should create a file called "init.sqs" in the mission's folder. for example, if the radio to have hidden is Bravo, put:
2 setradiomsg "NULL"
Now you won't see it when you start the mission. Since you want the radio to be avaliable when a group reach a waypoint, put in the waypoint's On Activation folder: 2 setradiomsg "Whatever you want".
Now, the radio channel will be activated.
Hope it helps