As discussed under Jacob 5:13, the critical text will restore all original instances of the modal might, even in cases where this modal does not have any conditional sense. Here in Alma 22:7, Joseph Smith replaced the past-tense form might with the present-tense form may, but there is nothing inappropriate about the use of might in this case.
Summary: Restore the modal might in Alma 22:7: “that they might assemble themselves together”.