Or HaChaim on Leviticus 8:4/אור החיים על ויקרא ח׳:ד׳

1 והקהל העדה, the community was assembled, etc. Perhaps when the people saw Moses taking Aaron, his sons, and the priestly garments, they assembled spontaneously. It is also possible that the verse tells us that G'd ordered the people to assemble around the entrance to the Tabernacle as explained by Vayikra Rabbah 10,9 on the word הקהל in verse 3 and that our verse reports that the people did so.