They must serve you ... They must join you

"They" refers to members of the tribe of Levi; the word "you" is singular and refers to Aaron. (See: rc://en/ta/man/translate/figs-you)

not come near you. You must take responsibility

Here "you" is plural and refers to both Aaron and the rest of the Levites. (See: rc://en/ta/man/translate/figs-you)