וּֽלְמַחְלְקוֹת֙ הַכֹּהֲנִ֣ים וְהַלְוִיִּ֔ם וּֽלְכָל־מְלֶ֖אכֶת עֲבוֹדַ֣ת בֵּית־יְהוָ֑ה וּֽלְכָל־כְּלֵ֖י עֲבוֹדַ֥ת בֵּית־יְהוָֽה׃
also for the courses of the priests and the Levites, and for all the work of the service of the house of the LORD, and for all the vessels of service in the house of the LORD:
Rashi
engage in the work i.e., in their work; this one to sing, and that one to guard the gates, every man to his work, as David prepared it.