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

class Company
{
    public Company(List<User> users) { this.Users = users; }
    public List<User> Users { get; set; }
}

class User
{
    public User(string name) { this.Name = name; }
    public string Name { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        List<Company> companies = new List<Company>();
        Company company1 = new Company(new List<User>(){ new User("first user"), new User("second user") });
        Company company2 = new Company(new List<User>() { new User("third user"), new User("fourth user") });
        companies.Add(company1);
        companies.Add(company2);

        companies = companies
            .Where(company => company.Users.Any(user => user.Name.Contains("third")))
            .ToList();

        foreach (Company company in companies) {
            Console.WriteLine(string.Join(", ", company.Users.Select(user => user.Name).ToArray()));
        }
    }
}