The original text had the subject pronoun they here. In his editing for the 1837 edition, Joseph Smith grammatically corrected the printer’s manuscript from they to what seems to be them. He also changed which to who, typical of his editing for the 1837 edition. The change to who is clear, but Joseph’s handwriting is quite uneven for the change of they to them. In fact, it is possible that Joseph actually intended to write those instead of them. In my opinion, the correction looks more like them, which is how the compositor for the 1837 edition set it. The critical text will restore the original they here as well as the which (“unto they which shall be at Jerusalem”), despite the ungrammaticality of the subject form they as the object of the preposition unto. In each case of they, them, or those as the antecedent for a relative pronoun, the critical text will follow the earliest reading. For a general discussion of the grammatical issues here, see under pronominal determiners in volume 3; also see the discussion under which in volume 3 for the change of which to who.
Summary: Restore in Helaman 16:18 the original they as the object pronoun for the preposition unto; despite its ungrammaticality, the original text allowed such usage; also restore the archaic use of which for who in this passage.