anchor.diff:  
   + Response 204
   
  -## UserGroup Collection [/groups{?page,page_size,sort,user_id,include}]
  +## UserGroup Collection [/user_groups{?page,page_size,sort,user_id,include}]
   A collection of User Group resources.
   
   All collections add a meta attribute hash containing paging
  
positive.code: 
  + Response 204
  
  ## Add a link [/groups/{id}/links/users]
  Only links to users may be added.
  
  ### Add user links [POST]
  
negative.code:             {
                  "links": {
                      "memberships.user_group": {
                          "href": "/user_groups/{memberships.user_group}",
                          "type": "user_groups"
                      },
                      "memberships.user": {
  