RSVP's for an engagement party?

You can't get annoyed no matter how inconvenient and rude it is, people are people and there will always be those who can't manage to respond in a timely manner. Just call them individually and let them know that you haven't received their response and that you need to know whether they're available to come in order to make your arrangements. Escalating to an argument (in itself is rude) over this type of stuff will only make your relationships harder and perhaps result in less people celebrating your special occasion, be a kind, understanding hostess and suck up the extra work.