You have several options. The most flexible way is to sign up for American Express mebmership rewards promotions, because they are the most flexible in terms of sharing miles/credits. You can add all your family members as authorized users and transfer credits directly into their accounts.

Chase also has promotions going on for signing up for credit cards, however, that is less flexible, as each person needs to sign up for their own accounts.

You can find the various AMEX promotions and Chase promotions by doing a search.