"Who had been represented there?"
Just to add something to this discussion...I don't believe in English we distinguish between singlular and plural when asking who either. 'Who was represented there?' could be asking about a single person or many people. You would have to make the question more detailed to specify; for example, 'Which people were represented there' vs 'Which person was represented there'.
OK. I understand. But again, if you want to say "who had been represented there" and you mean many people then, however, you must say "wer war dort vertreten"??? OK, what about this case- Three students were represented in some evening. After a while the teacher is asking to the class "I know that three of you had been represented in that evening, tell me who had been represented there?". So in this case we say "wer war dort vertreten?"?.