How to Add a User to the Site Group Programmatically

Steps

• Open Visual Studio in your system.
• Select Console Applciation template and give as name .
• Add a Microsoft.Cleint Assembly refrence file in right side refrence tab in visual studio.
• Replace Program.cs with the source code  

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.SharePoint.Client;
using System.IO;
namespace CSOMCodeSamples
{
   class Program
   {
      static void Main(string[] args)
      {
         // ClientContext - Get the context for the SharePoint Site

         ClientContext clientContext = new ClientContext("http://servername/sites/CSOM/");
         // Get the SharePoint web
         Web web = clientContext.Web;
         User user = web.EnsureUser("Gowtham");
         // Get the specific site group by name
         Group group = web.SiteGroups.GetByName("CSOM Owners");
         // Add a user to the specific group
         group.Users.AddUser(user);
         // Execute the query to the server
         clientContext.ExecuteQuery();
      }
   }
}

Hit F5 and check the output.