Prepaid is a great savings. You can only have one photopass account, but you can link as many cards as you want to the account. So sharing with family shouldn’t be a problem. Just be aware that you can only add pictures to the CD that are taken after the pre-purchase.
Also, if you feel like you didn’t get enough pictures via photopass once you get home, you can ask for a refund. That’s what I had to do for my last trip - because we didn’t take one photopass picture - and they refunded my money without question. It’s actually in the payment terms/rules ( you can get a refund up to 90 days after purchase).