class Program
    {
        public class Records
        {
            public int Id { get; set; }
            public string Text { get; set; }
            public string Group { get; set; }
            public DateTime Date { get; set; }
        }

        static void Main(string[] args)
        {
            var records = new List<Records>
            {
                new Records {Id = 1, Text = "Some text 1", Group = "A", Date = new DateTime(2015, 01, 01)},
                new Records {Id = 2, Text = "Some text 2", Group = "A", Date = new DateTime(2015, 01, 08)},
                new Records {Id = 3, Text = "Some text 3", Group = "B", Date = new DateTime(2015, 01, 04)},
                new Records {Id = 4, Text = "Some text 4", Group = "B", Date = new DateTime(2015, 01, 09)},
                new Records {Id = 5, Text = "Some text 5", Group = "C", Date = new DateTime(2015, 01, 07)},
                new Records {Id = 6, Text = "Some text 6", Group = "C", Date = new DateTime(2015, 01, 06)}
            };

            var q = 
                from r in records
                group r by r.Group into rg
                let last = (from r in rg orderby r.Date descending select r).FirstOrDefault()
                where last.Date >= (from r in records where r.Group == "A" select r.Date).Max()
                select last;

            var result = q.ToArray();
        }
    }