Name: group-definition
Type: request
Purpose: >
  Defines ("creates") a new group.
Response: group-definition-confirmation
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Date: { required: true }
  Group-Data: { required: true, type: object, class: event }
  Group-Id: { type: identifier, class: group }