Albert Barnes Commentary Nehemiah 8:4

"And Ezra the scribe stood upon a pulpit of wood, which they had made for the purpose; and beside him stood Mattithiah, and Shema, and Anaiah, and Uriah, and Hilkiah, and Maaseiah, on his right hand; and on his left hand, Pedaiah, and Mishael, and Malchijah, and Hashum, and Hashbaddanah, Zechariah, [and] Meshullam." — Nehemiah 8:4 (ASV)

The 13 persons mentioned were probably the chief priests of the course (shift) which was at the time performing the temple service.