How to add users in organic groups using services Rest api? I'm having problem while adding users in organic group. Here I'm trying to add user in organic group called "Supervisor", I'm using following json and Rest api :

Json :
{ "title":"Supervisor",
  "type":"group",
  "og_group_ref": { "und": [{ "target_id": "1921" }]},
  "field_records": { "und": [{ "value": "5" }]}
}


Rest api :
http://localhost/dev/test/node/1921

but, above json and api is not working, am I using correct json and rest api ?
Install this module: https://www.drupal.org/project/og_services
Go to admin/structure/services/list/[MY-ENDPOINT]/resources
Enable at minimum the join resource
Flush all of Drupal's caches
Make a POST call to [MY-ENDPOINT]/og/join/[GROUP-NID]/[USER-UID]