Albert Barnes Commentary Deuteronomy 29:15

"but with him that standeth here with us this day before Jehovah our God, and also with him that is not here with us this day" — Deuteronomy 29:15 (ASV)

With him who is not here with us - that is, as the Jews explain, posterity, which throughout all generations was to be taken as bound by the act and deed of those present and living.