using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Employee> employees = GetEmployees().ToList();

        IEnumerable<IGrouping<string, Employee>> groupedEmployees = employees

            .SelectMany(employee => employee.Groups.Select(group => (employee, group)))

            .GroupBy(pair => pair.group, pair => pair.employee);

        foreach (IGrouping<string, Employee> group in groupedEmployees)

        {

            Console.WriteLine(group.Key);

            foreach (Employee employee in group)

            {

                Console.Write("* ");

                Console.WriteLine(employee.FirstName);

            }

            Console.WriteLine();

        }

    }

    static IEnumerable<Employee> GetEmployees()

    {

        yield return new Employee { FirstName = "TestName1", LastName = "TestName1", Groups = new[] { "Group1", "Group2" } };

        yield return new Employee { FirstName = "TestName2", LastName = "TestName2", Groups = new[] { "Group1", "Group3" } };

    }   

}

​

public class Employee

{

   public string FirstName {get;set;}

   public string LastName {get;set;}

   public IEnumerable<string> Groups {get;set;}

}