Participant
noun
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.
映画好きのパーティーの最年長参加者は、なんと百才だった。
Surprisingly, the oldest participant of the moviegoer party was 100 years old.
ある参加者が会議を混乱させてしまってね。さんざんだったよ。
Thanks to one participant, the meeting was thrown into chaos. It was a disaster.
コウイチのスピーチは、参加者達から心からの称賛を受けました。
Koichi's speech received genuine praise from the participants.