Past and Future Together

Rashi:"and [also] with those who are not here"
also with future generations

Baal HaTurim: "But with the one who is here"
The last letters of the four words in this phrase can be rearranged to spell the word "Torah".  

Meir Laaretz: The next verse states: "but also with those here, standing here today before the L-rd our G‑d." What does this mean? Verse 14 says that not with you alone is the oath being made, but as verse 15 continues: with those who are here, standing today. It doesn't make sense. It sounds like there are 3 groups (1) "you" the live humans who are being addressed and (2) "those who are standing today before the L-rd our G‑d " a different group than #1, and (3) "those who are not here with us today", the unborn generations.

Who is the 2nd group? The Arizal in Sha'rei HaGilGulim says that there are a group of souls who were created in the 6 days of creation who have never been born yet as human beings on the earth. These souls are "standing before G‑d (in heaven)", yet "Here today", somehow present on earth that day just to become included in the covenant. Their time to be born as human beings will occur in the 7th millennium. They are new souls awaiting their time to develop earthly consciousness, awaiting their first birth. (Biurei HaMekubalim Benigleh)

Lubavitcher Rebbe: Jews must stand firm and unshakable, though they are the "smallest of the nations." This can only be when they are bound and united together: "all of you".
How is it possible to form a true and enduring union of different individuals with different interests, ideas, aspiration?
The answer is 'Before G‑d, your G‑d" The union of all Jews is possible because as a matter of fact, they are already united by virtue of their souls, the divine soul, which is actually a part of G‑d above, and which is found in every Jew without exception. And the profound reflection: "Before G‑d" who is "Your G‑d" your strength and your life—must bring forth and actualize the union of one Jew and another, and of the individual with the community of our people, in the fullest measure.

Zohar Lech 90:
Rabbi Hiya said, Come and see: How great are the deeds that the Holy One, blessed be He, performs, because He creates and portrays human beings in a similar way to the world. Every day, the Holy One, blessed be He, creates a world by mating couples, according to their worth. And He forms the images of all offspring, before they come to the world.
The Holy One...showed Adam every generation and its leaders.
Come and see: Rabbi Shimon said that it is written: "This is the book of the generations of Adam". (Gen. 5:1) Did he actually have a book? This has been explained. The Holy One, blessed be He, showed Adam every generation and its leaders. But how did He show him? If you say that he saw through the Holy Spirit that they are destined to appear in the world, just like a person who sees through wisdom can tell the future of the world, it is not so. Rather, he saw them all with his eyes; he literally saw the image of their forms in the world with his eyes. Because since the day the world was created, all the souls that were to appear in future human beings stand before the Holy One, blessed be He, in the same image and form that they will have in the world.

In the same manner, after all these righteous people depart from this world, their souls rise and the Holy One, blessed be He, creates for each of them another form in which to be clothed, just as they were clothed and covered while in this world. Therefore, all the souls stand before Him, and so Adam saw them, with his eyes.

One might say that after he had seen them, they ceased to exist and disappeared, but all that the Holy One, blessed be He, does, remains in existence. So they stand before Him until their time comes to go down to the world. And according to this, it is written: "But with him that stands here." And this has been explained — all the human beings, who were destined to appear in the world in the future, were standing there.

This should be examined, because it is written: "and also with him that is not here..." which leads us to conclude that it refers only to those who will be born from those standing there. Therefore, it does not say, "standing with us this day." Instead, it is written: "with us this day". Truly, everyone was standing there, but because they were not seen by the eye, the verse reads "with us this day", even though they are unseen.

You might say, "What is the reason that they were not seen here — just as they were seen by Adam's own eye, it would have been more proper to be seen? Here, when the Torah was given to Israel, they were looking at a different mirror, in which they saw the supernal levels eye to eye. And they were craving to look upon the glory of their Master. Therefore they saw the supernal glory of the Holy One, blessed be He, Himself alone, and nothing else besides Him!
...all human beings who shall appear in the future in the world stand before the Holy One...
Accordingly, all human beings who shall appear in the future in the world stand before the Holy One, blessed be He, clothed in the actual images in which they shall eventually appear. Therefore, it is written: "Your eyes did see my unshaped flesh: for in Your book all things are written..." (Ps. 139:16) What is the reason? Because another supernal image was similar to this one. Thus, it is written: "and who is a rock (tzur), save our G‑d? Who is such a good "portrayer" (tzayar) that he is able to draw everything as does the Holy One, blessed be He!