They shall be joined unto thee— Alluding to Levi's name; see Genesis 29:34. Stranger, in this verse, does not signify only those of other nations and relations, but all those who were not of the tribe of Levi.
Related Commentaries of Numbers 18:4
Numbers 18:4
4 And they shall be joined unto thee, and keep the charge of the tabernacle of the congregation, for all the service of the tabernacle: and a stranger shall not come nigh unto you.