Rashi on Deuteronomy 29:9/רש"י על דברים כ״ט:ט׳

1אתם נצבים YE ARE STANDING THIS DAY [ALL OF YOU BEFORE THE LORD] — This teaches that Moses assembled them in the presence of the Omnipresent on the day of his death, in order to initiate them into a covenant with Him.

2ראשיכם שבטיכם means as much as ראשיכם לשבטיכם YOUR HEADS OF YOUR TRIBES.

3זקניכם ושטריכם YOUR ELDERS AND YOUR OFFICERS — The more prominent were in front (i.e. they stood before the Lord in the order of their rank), and afterwards: “all the men of Israel”.