Here the word "them" refers to the believers; the word "his" refers to each person in the multitude. Verse 5 gives background information about the large number of Jews who were living in Jerusalem, many of whom were present during this event. (See: [[rc://en/ta/man/translate/writing-background]])
"every nation in the world." The word "every" is an exaggeration that emphasizes that the people came from many different nations. Alternate translation: "many different nations" (See: [[rc://en/ta/man/translate/figs-hyperbole]])