"And also the priests who draw near" (Exodus 19:22): one might think the priests are included among the people; Scripture says, "Go to the people" (Exodus 19:10)—the priests are not included among the people, as it is said, "and for the priests and for all the people of the assembly he shall make atonement," and it says, "And this shall be the priests' due from the people" (Deuteronomy 18:3)—the priests are not included among the people. And who are these priests? Nadab and Abihu. And when it says "and also," this includes the elders along with them.
Even the Priests Who Draw Near Must Keep Back
Yalkut Shimoni on Torah 284:3
(שמות יט כב) וְגַם הַכֹּהֲנִים הַנִּגָּשִׁים, יָכוֹל הַכֹּהֲנִים בִּכְלַל הָעָם, תַּלְמוּד לוֹמַר (לעיל פסוק י) "לֵךְ אֶל הָעָם" אֵין הַכֹּהֲנִים בִּכְלַל הָעָם, שֶׁנֶּאֱמַר "וְעַל הַכֹּהֲנִים וְעַל כָּל עַם הַקָּהָל יְכַפֵּר", וְאוֹמֵר (דברים יח, ג) "וְזֶה יִהְיֶה מִשְׁפַּט הַכֹּהֲנִים מֵאֵת הָעָם", אֵין הַכֹּהֲנִים בִּכְלַל הָעָם, וְאֵיזֶה הֵן הַכֹּהֲנִים, נָדָב וַאֲבִיהוּא, וּכְשֶׁהוּא אוֹמֵר וְגַם, אַף הַזְּקֵנִים עִמָּהֶם.