Name: device-group-joining-request
Type: request
Purpose: >
  Add another device to an existing device group
Response: device-group-details
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Device-Group-Id: { type: string, min_length: 32, max_length: 128 }
  Public-Key: { type: public_key }
Notes: >
  The sender ID must start with a valid device name and end with
  "@devgrp".
Rejection-Codes:
  22: Invalid sender
  41: Device group does not exist
  42: Device group is not open for new devices