Who is "Es Kol ha'Eidah" referring to?
Rashi: It refers to the entire congregation - whom Korach spent the night inciting against Moshe. 1
Ramban: It refers, either to the leaders of the congregation, who always assembled at the entrance of the Ohel Mo'ed, or the Bechorim, who were fit to perform the Avodah. 2
How did Korach gather the entire nation?
Rashi: The entire night he went from tribe to tribe, mocking Moshe, by explaining to the people that he was fighting, not only for himself, but on behalf of all of them 1 - to counter Moshe for taking the kingship for himself and the Kehunah for his brother, until he won them all over.
Targum Yonasan: He won over the people with his vast wealth. 2
In what form did the K'vod Hashem appear "?
What is the significance of the Pillar of Cloud appearing at that moment to all the people?
Targum Yonasan: It came in defense of Moshe and Aharon, whom Korach wanted to drive out of this world with his wealth.
Moshav Zekenim, citing R. Eliezer of Garmaiza: Hashem appeared Himself to punish them, lest people say that Moshe killed them with the Shem ha'Meforash. Even so, it did not help