As their brothers the descendants of Aaron did, they also cast lots in the presence of King David and of Zadok, Ahimelech, and the heads of the families of the priests and Levites—the family heads and their younger brothers alike.
Most of the Levites here named were mentioned before, Ch1 23:16, etc. They were of those who were to attend the priests in the service of the house of God. But they are here mentioned again as heads of the twenty-four courses of Levites (and about so many are here named), who were to attend the twenty-four courses of the priests: they are therefore said...
Commenting on 1 Chronicles 24:20-31
These likewise cast lots over against their brethren, the sons of Aaron,.... These, being divided into twenty four classes also, cast lots, which class should answer to, attend upon, and serve the first course of the priests, and which the second, and so on.
These likewise cast lots over against their brethren the sons of Aaron in the presence of David the king, and Zadok, and Ahimelech, and the chief of the fathers of the priests and Levites, even the principal fathers over against their younger brethren. (f) That is, every one had that honour which fell to him by lot.