参加 and 出席: joining activities and attending
Compare taking part in an activity with being present at an arranged gathering.
Taking part in an activity
参加 is read さんか and means taking part in an activity or event. 「マラソン大会に参加しました」 means that the speaker participated in a marathon event.
「オンラインでも参加できます」 says that participation is also possible online. 参加 does not require being physically present in the same place.
Being present at an arranged gathering
出席 is read しゅっせき and means attending a class, meeting, ceremony, or similar gathering. 「会議に出席しました」 means that the speaker attended the meeting.
「出席を確認します」 means checking attendance. An attendance record alone does not establish that the person presented or spoke actively.
Different focuses for the same meeting
Both 「会議に参加する」 and 「会議に出席する」 are possible. The first centers on taking part, while the second centers on attending the arranged gathering.
Do not define 出席 as doing nothing. Someone can attend and debate; avoid adding assumptions about actions that the word itself does not emphasize.
Does 「参加したいですが、まだ申し込んでいません」 describe someone who has already participated?
Show the answer and why
No. The speaker wants to participate but has not yet applied. Preserve the distinction between the desire in 参加したい and the incomplete application.