You are standing (upright) this day, all of you, before your God יהוה —your tribal heads, your elders, and your officials, every householder in Israel,
Behold, it is taught by the sages of Israel -
הַיּוֹם: דָּא (מ''ד א') רֹאשׁ הַשָּׁנָה,
When it says 'today' (in the opening verse), its speaking about Rosh HaShana!
And behold today the divine beings presented themselves before the LORD, and the Adversary-a came along with them.
And behold, it is known that all the power and strength of Israel comes from our unity, and the love that rests between them. As we find referenced at the giving of the Torah at Mount Sinai when Israel says the following -
Then he took the record of the covenant and read it aloud to the people. And the people said, “All that יהוה has spoken we will faithfully do!”
However, it is obvious that when someone gives charity, we don't say that only his hand was the giver of the charity and not himself, rather the commandment of charity is something attached to the whole essence of the person, since his whole body is considered one entity, and therefor the commandment he fulfilled is on his whole body. This is the same with the people of Israel, we must be like one with each other, each one with his friend, and then we will be considered as one body. And through acting this way, all the commandments that you fulfill will actually be considered as it your friends did it as well, and whoever loves Israel with a complete love, its fitting to say on him that he fulfills in one day thousands upon thousands of commandments and acts of kindness through his unity he has with his brethren that are doing those mitzvot.
And behold, when Israel says everything that HaShem says we will do, which happens prior to the Israelites receiving the Torah, they were accepting unity upon themselves unity and love between the individual and his/her fellow - as it says -
ויחן שם ישראל. כְּאִישׁ אֶחָד בְּלֵב אֶחָד,
ויחן שם ישראל AND THERE ISRAEL ENCAMPED as one man and with one mind
And through unity, we are worthy to be saved from the Satan and harmful happenings. As we can see with Haman the evil one who was not able to bring accusation upon Israel and to rule over them until he saw that there was dissension among them. This is what is taught by the sages based on what Haman says below -
Haman then said to King Ahasuerus, “There is a certain people, scattered and dispersed among the other peoples in all the provinces of your realm, whose laws are different from those of any other people and who do not obey the king’s laws; and it is not in Your Majesty’s interest to tolerate them.
“Go, assemble all the Jews who live in Shushan, and fast in my behalf; do not eat or drink for three days, night or day. I and my maidens will observe the same fast. Then I shall go to the king, though it is contrary to the law; and if I am to perish, I shall perish!”
שׁוֹשַׁנַּת יַעֲקֹב צָהֲלָה וְשָׂמֵחָה בִּרְאוֹתָם יַחַד תְּכֵלֶת מָרְדְּכָי:
The rose of Jacob cheered and rejoiced when they [all] saw Mordechai dressed in royal blue.
He Who performed miracles for our fathers, and redeemed them from slavery to freedom, may He redeem us soon and gather our dispersed from the four corners of the earth; united in friendship is all of Israel! And let us say, Amein.
And to answer our first question, the word Netzavim (upright), is a language of standing with strength, as we see it used elsewhere in this manner -
וְ֠הִנֵּ֠ה אֲנַ֜חְנוּ מְאַלְּמִ֤ים אֲלֻמִּים֙ בְּת֣וֹךְ הַשָּׂדֶ֔ה וְהִנֵּ֛ה קָ֥מָה אֲלֻמָּתִ֖י וְגַם־נִצָּ֑בָה וְהִנֵּ֤ה תְסֻבֶּ֙ינָה֙ אֲלֻמֹּ֣תֵיכֶ֔ם וַתִּֽשְׁתַּחֲוֶ֖יןָ לַאֲלֻמָּתִֽי׃
There we were binding sheaves in the field, when suddenly my sheaf stood up and remained upright; then your sheaves gathered around and bowed low to my sheaf.”