SL HomeschoolMom

Hi everyone. My husband and I have booked and paid for a Balcony Dinner for us on-line. We are in a Caribe Balcony on the Crown. We have another couple cruising with us in an inside cabin. I would like to invite them to join us. I'm sure there would be room on our balcony. My question is can I wait until we board to add them to our Balcony Dinner so that I can use my OBC to pay for them? Or do I have to change my booking and make it for four now and pay with my credit card? Thanks!