Add a user to a particular group who already exists in the site collection using Client Object Model

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using Microsoft.SharePoint; 
using Microsoft.SharePoint.Client; 
namespace ClientObjectModel 
{ class Program 
{ static void Main(string[] args) 
{ //Add a user to a particular group who already exists in the site collection string siteURL = "http://serverName:1111/sites/SPSiteDataQuery/"; ClientContext context = new ClientContext(siteURL); GroupCollection groupColl = context.Web.SiteGroups; Group group = groupColl.GetById(7); User user = context.Web.EnsureUser(@"DomainName\UserName"); 
group.Users.AddUser(user); 
context.ExecuteQuery(); 
} 
} 
}