According to most opinions, including Rashi, Ibn Ezra, Lekach Tov, M’nos HaLevi, R’ Moshe Dovid Valle, and the Vilna Gaon, “the ones who join” are future converts.
R’ Chaim Kanievsky wonders why Purim is different from other mitzvos that converts need to be mentioned specifically regarding Purim’s celebration. He answers that, even though converts were not party to the miraculous rescue, one’s descendants should be obligated to recite a Thanksgiving Blessing for one’s rescue, as they were affected by it, as well. This is similar to a student showing gratitude for the rescue of his rebbe. Had it not been for the rebbe’s being rescued, the student would not have had access to the World to Come. For this reason, although converts did not experience the miracle of the first Purim, their new people’s having gone through it is reason enough for them to accept the mitzva of celebrating the holiday.
According to the Ateres Moshe, converts are mentioned here to mirror Moshe’s statement (Devorim 29:14) that the acceptance of the Torah applies equally to those who were there and even those not there.
In the view of the Midrash Shmuel, converts are not always sincere about their reason for joining the Jewish people. Those who converted in Persia (Esther 8:17), for instance, may have done so in order to save their lives. However, in commemoration of the Persians who converted sincerely then, Purim was accepted as a way to celebrate future sincere converts, as well.
The Maharal adds that a convert can’t ignore even one rabbinic law, and rabbinic law is a motif throughout Megillas Esther.
In explaining how ora (“light”) represents Torah, the Ben Ish Chai writes that ora is written with a hey because it means ohrhey, or the light of H-Shem.
Rav Tzaddok HaKohen writes that ora is written with a letter hey because the verse intends it to be feminine since the Torah being described here is specifically Torah she’bal peh (“the Oral Law”). As Rashi (on Mishlei 1:8) writes, the Torah she’bal peh is represented by the feminine. Rav Mordechai Gifter explains that this is because the rabbis know the natural foibles of their people in the same way that a mother considers the nature of her son.
From the time the Jews ignored Mordechai (the leading rabbi of the generation) by attending Achashverosh’s party until they re-accepted the Oral Torah with the words (Esther 9:27) “kimu v’kiblu” (“they took and they accepted”), the Jews of that period were struggling with Torah she’bal peh, and its necessary rabbinic accompaniments.
Similarly, the Midrash Yerushalmi interprets yikar as denoting the judges, who were also the rabbis.
Midrash Chaseros v’Yitaros writes that sasson (“joy”) is spelled incomplete (without a vuv) because no joy can be complete until Moshiach comes and the Beis Hamikdash is rebuilt, bimheira biyameinu.
R’ Chaim Kanievsky writes that it is written incompletely because circumcision, which this represents, has an element of pain. He notes that sasson is spelled completely in the next verse (Esther 8:17) because we should strive to add to the joy of Purim as though nothing is missing, as the Halacha (Biur Halacha 695, dh “ad d’lo yada”) states explicitly regarding the custom to become inebriated on Purim.
In his commentary on this verse, the Vilna Gaon writes that the expressions used in this verse refer to the four components of man – the nefesh (animus, ability to move), the ruach (life force), the neshama (spiritual soul), and the guf (physical body). He writes that the nefesh and guf are really one, as according to the Zohar which writes that “the nefesh is a partner to the guf.” Haman wanted to destroy every part of the Jew; he therefore used three expressions to denote his real intent. However, the neshama is fed by performance of mitzvos, the ruach is fed by pleasure, and the guf is fed by food. The Vilna Gaon writes that this is the reason for the three mitzvos of Purim; publicly reading the Megillah (see Esther 8:26) feeds the neshama, having joy (see Esther 8:17) feeds the ruach, and feasting (ibid.) literally feeds the guf. There is also an extra mitzvah of giving gifts (ibid. 19). Perhaps Haman’s desire to plunder the Jews’ wealth is the reason we use our resources for this last mitzvah.
R’ Dovid Chadida writes similarly that by using the word “l’hashmid” (“to destroy”), Haman intended to kill the Jews spiritually, by using the word “l’harog” (“to kill”), Haman intended to kill the Jews’ physically, and by using the word “l’aveid” (“ to annihilate”), Haman intended to kill the Jews’ financially. In the verses of Shema (Devarim 6:5), these are the very three things with which Jews are expected to dedicate themselves to love H-Shem.