I agree with the above posters, if you want to save the most possible, book everyone individually. Also booking all the flights as one way could also help you save money in the long run.
After you book, check back once a week or so, to see if they flights have dropped in price, if so, just go through the change ticket process, its fairly quick. If you paid cash for your ticket, you will get a travel voucher to be used on a later flight. If you paid with points, you will get the point difference put back in your account.
The reason to book them all as one ways is due to the rebooking process. If you book it as a round trip, and one way decreases in cost, but the other way increases in cost, they will re-book both ways, and you might not save anything. If you book one way tickets, you can just rebook the way that lowered in price, while not worrying about the way that increased.