Name: membership-admission
Type: request
Purpose: >
  Make an identity member of a particular group
Response: membership-admission-confirmation
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Date: { required: true }
  Group-Id: { required: true, type: identifier, class: group }
  Participant-Id: { required: true, type: identity }