There lies the confusion. I am aware that a person "participants in" an activity. However, when you have a statement such as, "This is a comment from a participant of the Nat'l. Teleconference," to me "of" sounds more appropriate and "in" sounds awkward.
Not to me. 'Of' sounds awkward, and 'in' appropriate. I am a university-educated, upper middle-class Canadian-American native English speaker. There may be (!) other opinions.