This endpoint adds a user or group as a graph viewer.
{
    "data": [
        {
            "attributes": {
                "first_name": "Bertram",
                "last_name": "Gilfoyle",
                "profile_phrase": "",
                "reputation": 1,
                "status": "active",
                "user_since": 1530008602
            },
            "id": "gilfoyle",
            "links": {
                "self": "https://www.virustotal.com/api/v3/users/gilfoyle"
            },
            "type": "user"
        }
    ],
    "links": {
        "self": "https://www.virustotal.com/api/v3/graphs/g5598743d5f3c699f1c90e76c2c2d541d41f1b56bd8114c06d181ed3f60cfcada/viewers?limit=10"
    }
}