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That depends very much on what routes you would like to redeem miles for in the future. A single roundtrip in Coach will not earn very many miles per passenger. However, there are a small number of frequent-flyer programs that let you open a "family" or "household" account, and pool everyone's miles to have enough for one or more redemptions. Some programs assess very high cash surcharges on some redemptions; others do not.
Do you plan to take advantage of any of the numerous mileage bonuses offered for credit-card sign-ups?
Tell us a little more, and you'll get some good advice here.