def METHOD_NAME(self, device, GrpId):
    """
    add a device ( NwkId, ep, ieee) to a group
    """
    if GrpId not in self.ListOfGroups:
        self.ListOfGroups[GrpId] = {}
        self.ListOfGroups[GrpId]["Name"] = ""
        self.ListOfGroups[GrpId]["Devices"] = []
    if device not in self.ListOfGroups[GrpId]["Devices"]:
        self.ListOfGroups[GrpId]["Devices"].append(device)