Add an existing Feed to a Group

{
  "type": "object",
  "properties": {
    "feed_key": {
      "type": "string"
    },
    "group_key": {
      "type": "string"
    },
    "username": {
      "type": "string"
    }
  },
  "required": [
    "group_key",
    "username"
  ],
  "additionalProperties": false
}