code: func (n *NetworkServerAPI) AddDeviceToMulticastGroup(ctx context.Context, req *ns.AddDeviceToMulticastGroupRequest) (*empty.Empty, error) {
  	var devEUI lorawan.EUI64
  	var mgID uuid.UUID
  	copy(devEUI[:], req.DevEui)
  	copy(mgID[:], req.MulticastGroupId)
  
  	if err := storage.AddDeviceToMulticastGroup(storage.DB(), devEUI, mgID); err != nil {
  		return nil, errToRPCError(err)
  	}
  
  	return &empty.Empty{}, nil
  }
docs: // AddDeviceToMulticastGroup adds the given device to the given multicast-group.