List<Person> lists = new List<Person>();
            lists.Add(new Person("123"));
            lists.Add(new Person("456"));

            var qs = lists.Where(l => l.group == "123");
            foreach (var q in qs)
            {
                Response.Write(q.group);
            }

public class Person
    {
        public string group;
        public Person(string group)
        {
            this.group = group;
        }
    }