If i understand you correctly...
Try this:
DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Group", typeof(string)));
dt.Columns.Add(new DataColumn("SubGroup", typeof(int)));
dt.Rows.Add(new object[]{"A", 1});
dt.Rows.Add(new object[]{"A", 2});
dt.Rows.Add(new object[]{"B", 3});
dt.Rows.Add(new object[]{"B", 4});
dt.Rows.Add(new object[]{"C", 5});
dt.Rows.Add(new object[]{"C", 6});
string dropdownselecteditem = "A";
var result = dt.AsEnumerable()
.Where(x=>x.Field<string>("Group")==dropdownselecteditem);
foreach(var record in result)
{
Console.WriteLine("{0}", record.Field<int>("SubGroup"));
}
Result:
1
2