Don't Know Who

"We'll meet again, don't know where, don't know when ... " So, who were all those people singing along with Stephen Colbert for his show's grand finale? (video): http://www.slate.com/blogs/browbeat/2014/12/19/colbert_report_finale_annotated_list_of_everyone_who_appeared_in_sing_along.html
   The song "We'll Meet Again," btw, is from 1939, the first year of World War II. It was sung by Dame Vera Lynn (video): https://www.youtube.com/watch?v=R9Tw16dNyJs