Brinker, always the self-righteous leader when there's a way to make himself more important in the eyes of his classmates, organizes the gathering at the First Building to determine the true and complete story of the events that led up to Finny's fall from the tree during the Summer Session.
Brinker assumed the lead role as director, organizer, and questioner, imagining himself as a very capable investigative lawyer. Gene and Finny were there; roughly "ten members of the senior class sat on the platform; all of them were wearing their black graduation robes" to act as the jury, and another "eight or ten" students were seated on benches facing the front of the room. The members of the jury and the witnesses are not identified except for Leper.
It is reasonable to assume, however, that Chet Douglas and Bobby Zane were probably among the students present that night, as they shared many experiences with the others throughout the story.