The Literal Commandment
But bring with you also your brothers, the tribe of Levi, the tribe of your father, that they may be joined with you and serve you, while you and your sons with you are before the tent of the testimony. (Leviticus 19:30)
This commandment is literally for Aaron and his sons (the Aaronic priests) to be "before the tent of the testimony" (the Tabernacle). The Hebrew word that might suggest "guarding" [shamar] is not present in this verse. Their presence, however, might suggest guarding in order to prevent injury or harm to those who are improperly prepared to come into the holy precinct of the Tabernacle.