Jamieson, Fausset and Brown on Numbers 16:3

3. they gathered themselves together against Moses and against Aaron--The assemblage seems to have been composed of the whole band of conspirators; and they grounded their complaint on the fact that the whole people, being separated to the divine service (Ex 19:6), were equally qualified to present offerings on the altar, and that God, being graciously, present among them by the tabernacle and the cloud, evinced His readiness to receive sacrifices from the hand of any others as well as from theirs.