Personally I woulnd't pre-register the card they sent you to use and here's why..
If you preregister the card as soon as you get photos taken on the card (or within an hour or two) those photos will start appearing in your account and your 30 day ordering clock will start. Meaning you will only have 30 days to do your edits/enhancements on your photos before you order your CD. There is no reason to enter the card before you are ready to do your edits.
Say for example you arrive at WDW on July 28th and start getting photos taken. You will then have until August 27th to place your order before your photos start to disappear from your account if you use a pre-regsiter card. If you just use the card they sent you, or any cards they give you in the parks, then you will have until August 27th to ENTER your pictures (taken on July 28th) and you'll have until Sept. 27th to place your order. An extra 30 days.
Basically, if you do it right you will have a total of 60 days to do your edits before you have to place your order. You must download your pictures by 30 days from date taken and then you must order within the next 30 days. In this way I have managed to maximize my CD orders, every time. I have even been able to combine my Fall WDW trips with trips to DLR after and get the DLR price of $59.95 instead of the $99.95 prebuy price.
As PP have stated, you can have unlimited number of cards. Every group can have their own. If you forget the one you had, you can get a new one. You can have unlimited photos in your account once you complete your edits. The most CDs I have had, in one order, are 5.
If you have any more questions, please feel free to ask. I consider myself a PP CD expert.
All of your family's photos can be on one CD order and then you can just burn them copies (be sure to curn them a copy of the Copyright agreement) for their use.
Have fun!
(Actually if you wanted to have 2 separate accounts with all the same photos you can do it-you would take all your cards to the PP store and ask them to copy them onto each other's cards. Though why anyone would do this I don't know-it would mean you would then place 2 CD orders. No reason to since you can just copy to one you get to share.)
Oh, and here's a final tip for those who may have those cute wrist PP cards and want to reuse them-DON'T if they are from a previous trip! I did this a few years back, not thinking, and once the photos were taken the appeared in the account I had entered that card in the year before. Problem was it wasn't my account, it was back in the day when you could do CD Shares on the boards and the account belonged so someone else!
Luckily I contacted the owner of the account and Disneyphotopass and they were able to transfer the photos to my account.
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