You don't get their miles, each person will earn mileage on their own respective FFP account. The credit card program (citibank premier pass card) is separate, it enables you earn flight points (an instrument within citi thankyou points program), from all tickets you paid using your card. If you have the regular Premier Pass card, you earn 1 flight point for every 3 miles they fly, and flight points are typically awarded automatically after your trip. If the flight points do not post to your credit card statement (give it a minimum of 4 or 6 weeks after your trip), then call the credit card company. In the past, people had been asked to provide boarding pass to prove their travel, so make sure your family members keep their boarding passes until after all credits (airline FFP and your TYP) are deposited.

Be aware that, in the thankyou program, flight points need be matched by purchase points in order to become regular thank you points that get transferred to your thank you account - not matched flight points would remain in your credit card account for future use.

There are many posts about thank you points, please do a search in the miles buzz and other credit cards forums for many useful insights.