If your dining arrangements are not what you want, go to the Dining room on your first afternoon on board- the Maitre d' is usually there to take requests for changes.
You contact your original travel agent and instruct them to link your bookings with your friends and tell them you all want to dine on the same table. Once you are on board just check with your friends that you are all on the same table, there will be a card in all your rooms telling you which sitting and table you have been given. If they are not the same just go down and inform the head waiter of HIS error and he should accommodate you. Just make sure you get verification in writing from your travel agents to take with you. Even if you have all booked with different travel agents the bookings can be linked, as we often meet up with some friends from America on Celebrity Cruises and we have always had the same table as them.