A high percentage of people who attended the event were married couples
A large portion of attendees were married couples
色々な言い方はありますが、一番目とニ番目をどちらでも使えます。
a high percentage, a large portion, a large percentageでもいいです。
people who attended the event=attendees
もしどのイベントがわからない人と話す時、people who attended the eventを使った方がいいと思います。
ご参考になれば幸いです。