I know you can split the cost with linked accounts on MDE but will they be able to download the pictures with their log in or will they have to use mine?
They have to use yours. That's why many people assign the Memory Maker to a dummy account that everyone "friends" and then everyone has that login.
how do you set up a dummy account?
Create a new account with a different name, a different email address, and your same postal address. I used my sister-in-law's name (she's not going). Then from that account, friend your real account and all the adults you want in your group. Then buy the memory maker and assign it to this new account (that will be an option when you purchase it. All the modifying and downloading of pictures will happen from this new account, so you have to give the username and password out to your group.