“You stand here today all of you.” This means that he gathered them… to admit them into the covenant (RASHI). “All of you” means that everyone entered into the Arvut [mutual responsibility] (Ohr HaChaim).
There is a question why he begins with plural form, “all of you,” then shifts to singular form, “every man from Israel.” It means that “all of you” permeates everyone in Israel, meaning that every person from Israel will be included with “all of you,” as it is written, “And the people camped at the bottom of the mountain,” as one man with one heart. In other words, when there is love of Israel, they can succeed, as it is written, “Ephraim is joined to idols; let him be.”
Man is a small world and comprises the entire world. He should achieve the degree of being singular, as it is written, “Rewarded, he sentences himself and the entire world to the side of merit” (Kidushin 40b).
Therefore, when a person admits all the individuals, “your heads and your tribes,” for everyone must join in the covenant, meaning to come to be a worker of the Creator “with all your heart—with both your inclinations.” In other words, even the lowly attributes in man should undergo correction.
“So that He may establish you today as His people and He will be a God unto you.” That is, a person will achieve this attainment, that he is in the singular authority.