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();
}
}
}