using System;
using System.Linq;

public class Test
{
	public static void Main()
	{
		var listOfGroups = new[] { "a1", "a2", "b1" }
		                       .GroupBy(x => x.Substring(0, 1))
		                       .ToList();
		
		var newGroup = new[] { "foo", "bar" }.GroupBy(x => "baz").Single();
		
		listOfGroups.Add(newGroup);
		
		foreach (var group in listOfGroups)
		{
			Console.WriteLine(group.Key + ":");
			Console.WriteLine("    " + string.Join(", ", group));
		}
	}
}