Trouble with Private lines.
For me, it was a question of scrolling. When I just tested it, the bottom of the list appeared and was populated by all of the language groups; the individual players were up at the top of the list. If that doesn't work, you can type it manually. Just open up a private line to one player, and add the rest you want to include using a semicolon and a space between them "; ". I know that it might take longer, but it works.