yes, I don't get this at all. What's wrong with checking in yourself, online, 24 hours before your flight? If you do that, you'll most likely get into the A group. And I don't know how they guarantee that your family will sit together. Somebody posted on this board once that they were traveling with 15 family members and wanted them all to sit together. Suppose there were a lot of travelers on a continuing flight and they'd taken all the seats that would work for this group of 15? Then what? I'd bet all it does is give the group another means to argue with the flight attendants: "I paid to have my group sit together and I want these people moved!"
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