What is the meaning of "va'Yosef"?
Rashi: He additionally [gathered people. They previously gathered in Chevron to make him king]. Radak 1 - there were already many with him. Pelishtim fought after he became king, and this is when he brought the Aron, like is proven in Divrei ha'Yamim I, Perakim 13-14.
Radak: This is like va'Ye'esaf (he gathered) from the root Asaf (with an Aleph). Also "va'Yocher Min ha'Mo'ed" (20:5) is from the root Achar. Also "Tosef Rucham Yigva'un" (Tehilim 104:29) is like Te'esof.
Radak: Also Yonason translates like this (v'Osif).
What is the meaning of "Bachur"?
R. Yeshayah, Malbim: It is chosen. They were the Chachamim.
Here it says 30,000. Why do Bamidbar Rabah (4:20) and the Yerushalmi (Sanhedrin 10:2) say that he gave Semichah to 90,000 Chachamim that day?
Malbim: We expound "Es Kol Bachur"; each word adds the same amount. It seems that every congregation sent a Beis Din of three.
