Jamieson, Fausset and Brown on Exodus 24:9

9. Then went up Moses, and Aaron--in obedience to a command given (Ex 24:1, 2; also Ex 19:24), previous to the religious engagement of the people, now described.

Nadab, and Abihu--the two oldest sons of Aaron [Ex 6:23].

seventy of the elders--a select number; what was the principle of selection is not said; but they were the chief representatives, the most conspicuous for official rank and station, as well as for their probity and weight of character in their respective tribes.