Kind of new to Demozoo, saw that a group release was missing and tried to add it. After adding I saw a much longger list of "MEMBER PRODUCTIONS" below the "PRODUCTIONS" list, and the release was already there.

What is the difference between the two, and what makes one production go into one and not the other?

https://demozoo.org/groups/12953/

Quite simple - if it's listed as "person / group" then it's a member release, if it's listed as "group" then it's a group release.

We generally prefer at least executable things to be "group" and then the person involved gets added as a credit on that production, but it's down to the person that adds the information.