using System;

using System.Linq;

using Newtonsoft.Json.Linq;

​

public class Program

{

    public static void Main()

    {

        var json1 = "[\r\n  {\r\n    \"Id\": 1,\r\n    \"GroupId\": 5,\r\n    \"Name\": {\r\n      \"First\": \"Tony\",\r\n      \"Last\": \"Stark\"\r\n    },\r\n    \"Phone\": \"1111111111\"\r\n  },\r\n  {\r\n    \"Id\": 2,\r\n    \"GroupId\": 5,\r\n    \"Name\": {\r\n      \"First\": \"Steve\",\r\n      \"Last\": \"Rogers\"\r\n    },\r\n    \"Phone\": \"2222222222\"\r\n  },\r\n  {\r\n    \"Id\": 3,\r\n    \"GroupId\": 6,\r\n    \"Name\": {\r\n      \"First\": \"John\",\r\n      \"Last\": \"Doe\"\r\n    },\r\n    \"Phone\": \"3333333333\"\r\n  }\r\n]";

        var json2 = "[\r\n  {\r\n    \"Id\": 5,\r\n    \"Description\": \"Avengers\"\r\n  },\r\n  {\r\n    \"Id\": 6,\r\n    \"Description\": \"Others\"\r\n  }\r\n]";

        var mainArray = JArray.Parse(json1);

        var groupArray = JArray.Parse(json2);

        var targetArray = new JArray();

        foreach (JObject o in mainArray)

        {

            var id = o.Value<int>("GroupId");

            var target = groupArray.FirstOrDefault(r => r.Value<int>("Id") == id);

            if (target == null)

                continue;

            var newObj = new JObject();

            newObj["Id"] = id;

            var name = new JObject(new JProperty("Last",o["Name"]["Last"]));

            newObj["Name"] = name;

            newObj["GroupDescription"] = target["Description"];

            targetArray.Add(newObj);

        }

​

        targetArray.ToString().Dump();

    }

}