Add user to the group in SharePoint 2010 using Client Object Model

 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.SharePoint.Client;
 
namespace COM
{
    class Program
    {
        static void Main(string[] args)
        {
            // siteURL is the string that contains the site URL 
            string siteUrl = "http://serverName:50000/sites/Testing";
            // ClientContext object is used to get the context for the SharePoint objects
            ClientContext clientContext = new ClientContext(siteUrl);
            Web web = clientContext.Web;
            User user = web.EnsureUser(@"domainName\userName");
            Group testingOwnersGroup = web.SiteGroups.GetById(6);
            UserCollection userCollection = testingOwnersGroup.Users;
            userCollection.AddUser(user);
            clientContext.Load(userCollection);               
            clientContext.ExecuteQuery();           
        }
    }
}