Albert Barnes Commentary 1 Chronicles 24:3

"And David with Zadok of the sons of Eleazar, and Ahimelech of the sons of Ithamar, divided them according to their ordering in their service." — 1 Chronicles 24:3 (ASV)

Zadok and Ahimelech (or rather, Abiathar; see 1 Chronicles 24:6) assisted David in drawing up the priestly courses, just as the captains of the host assisted him in making the divisions of the singers (1 Chronicles 25:1).