What does the Pasuk mean? From where were Moshe and the elders 'gathered'?
Rashi: The Pasuk means that they were gathered to their houses (they went home) 1 from the entrance of the Ohel Mo'ed.
Why does the Torah see fit to mention this?
Rashi: To teach us that Hashem only sent the punishment after all the Tzadikim had arrived home.