‘And Yahweh said to him, “Go, get yourself down, and you will come up, you, and Aaron with you, but do not let the priests and the people break through to come up to Yahweh, lest he break out upon them.” So Moses went down to the people and told them.'
Again Yahweh issues His warning through Moses. Whether priest or people none may approach Yahweh in the Mount except His two priests, Moses and Aaron. Otherwise they will face the dire consequences of which they have been warned. He is aware how easily someone might feel he was the equal of Moses and Aaron, especially among the priests, and might feel he could brave the ban. God did not want this to happen. He had no delight in the death of His people.
“And Aaron with you.” We are not told that Aaron goes into the mount with Moses until Exodus 24:1 where he is one of a group. But this need not mean that Aaron did not go up with Moses. We have regularly seen in the past that Yahweh has said something will happen and then it is assumed that it happened. The word of Yahweh was sufficient to establish the happening. Thus we may assume that often when Moses goes up, Aaron goes up. Compare how later Joshua seemingly accompanies Moses but nothing is said of his presence after the initial statement (Exodus 24:13).