This can't be done without code or a third-party tool like qRules / FormsViewer. That's because InfoPath doesn't have any rule actions for inserting new repeating elements. If you change your data structure to a repeating table and just have a single select dropdown and a single person, then have the user manually insert rows, that may be a work around. Then if the user needs to pick more than one item, she inserts another row and you don't have to worry about the repeating elements.
Hilary Stoupa