using System;
using System.Linq;
using System.Collections.Generic;

class Item{
   public string ItemName { get; set; }
   public string GroupName { get; set; }
}

public class Test
{
	public static void Main()
	{
            List<Item> webservice = new List<Item>() { 
                new Item(){ItemName="Item1",GroupName="A"},
                new Item(){ItemName="Item2",GroupName="B"},
                new Item(){ItemName="Item3",GroupName="B"},
                new Item(){ItemName="Item4",GroupName="A"},
                new Item(){ItemName="Item5",GroupName="A"},
            };
            var groupItems =
                webservice.Where(i => i.GroupName != null)
                          .GroupBy(i => i.GroupName)
                          .Select(grp => new { Group=grp.Key,Items=grp.ToList()});
            foreach (var groupItem in groupItems)
                Console.WriteLine("Groupname: {0} Items: {1}"
                    , groupItem.Group
                    , string.Join(",", groupItem.Items.Select(i => i.ItemName).ToArray()));	
	}
}