When you are not able to add the GoControl Z-Wave Plug-in Dimmer Module to a network:
When you are not able to add the GoControl Z-Wave Plug-in Dimmer Module (PD300Z-2) to a group then it may be because the Home ID and Node ID were not cleared from the group after testing.
So, for this you can RESET the unit using your controller so that you can remove it from the network.
Even when you add the dimmer to a group to include it in the network you cannot remove it from the network by just removing it from a group. Generally, when you remove the dimmer from a group, it only acts as a repeater.