Group by in c# for Document Directory having Column contain same value

I have Document Directory, in That I have Document Content type,and 1 more Columns Engagement , that columns have values comeing from term store.

Engagement Column value: AAA BBB CCC DDD EEE

so WHEN I upload Document that time 2 or more record have Engagement Column value are AAA or BBB . Now My Requirement is to Group by all record by Engagement Column value

My ref code:

List listItems = list.Items.Cast().ToList();

if (listItems.Count > 0) { var groupedItems = listItems.GroupBy(a => a[_EI_Field_Name]).ToDictionary(g=>g.Key);

foreach (var groupItem in groupedItems) { foreach (var groupValue in groupItem) {

  }
}