How many additional cards do you need? You can request "up to three" supplementary Platinum cards for the single $175 fee.

Do the additional cardholders need Platinum cards, or just the ability to make purchases? For $45 you could request "up to five" Gold cards.

Another approach for no annual fee you could open an American Express credit card account, such as Optima, Blue, The Knot or The Nest. You need to apply for the credit card in the same name as the charge card to link to the same Membership Rewards account. (Used alone the credit cards participate in the inferior Membership Rewards Express program, but when linked to a charge card they earn the same as a purchase made with Platinum.) The primary drawback is that the credit card would generate a separate statement.