Any node that is on the network can be seen by any other node within the network.
You can set the NJ to any value between 0 - 0xFF. When less than FF, it allows joining for a period of time.
You can also enable or leave joining at FF and then change it to 0 once all of the desired devices have joining. You can also use the NJ value to control who an end device joins with. By setting NJ to 0 one say the Coordinator and leaving NJ to a value other than 0, I can allow an end device to join the router. Yet as it is on the network then, any other node within the network can still access it.