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]])
Here the word "them" refers to the believers; the word "his" refers to each person in the multitude.
"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]])