using System.Collections.Generic;

using System.Linq;

using System;

​

public class Program

{

    public static void Main()

    {

        var userList = new List<User>();

        userList.Add( new User { UserID = 1, UserName = "UserOne", GroupID = 1 } );

        userList.Add( new User { UserID = 2, UserName = "UserTwo", GroupID = 1 } );

        userList.Add( new User { UserID = 3, UserName = "UserThree", GroupID = 2 } );

        userList.Add( new User { UserID = 4, UserName = "UserFour", GroupID = 1 } );

        userList.Add( new User { UserID = 5, UserName = "UserFive", GroupID = 3 } );

        userList.Add( new User { UserID = 6, UserName = "UserSix", GroupID = 3 } );

        var groupedCustomerList = userList

            .GroupBy(u => u.GroupID)

            .Select(grp => grp.ToList())

            .ToList();

        foreach(var groupList in groupedCustomerList)

        {

            foreach(var grut in groupList)

            {   

                Console.WriteLine(grut.GroupID);

                Console.WriteLine(grut.UserID);

                Console.WriteLine(grut.UserName);

            }

        }

    }

}

​

public class User

{

  public int UserID { get; set; }

  public string UserName { get; set; }

  public int GroupID { get; set; }

}