You know that 参加 is participation, so someone who does participation is a participant.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
Surprisingly, the oldest participant of the moviegoer party was 100 years old.
Thanks to one participant, the meeting was thrown into chaos. It was a disaster.
Koichi's speech received genuine praise from the participants.