Name: device-group-opening-request
Type: request
Purpose: >
  Open a device group for adding another device
Response: device-group-details
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Device-Group-Id: { type: string, min_length: 32, max_length: 128 }
Notes: >
  The sender of this message has to be a member of the device group,
  i.e. the ID must start with a valid device name and end with
  "@devgrp".
Rejection-Codes:
  22: Invalid sender
  31: Sender is not a member of the group