This glitch works off the fact that you are automatically placed onto the character that you were last selected on. To do it, have one person select the character so it says "OK", then have them deselect and move. Then have the another person (up to all three other people, in turn) who wants the same character select so that it reads "OK". Then, have controller one go back to the previous screen and once going back to the character selection screen, everyone who selected that character will be piled on the same one.