Does everyone in a group need to have unlimited storage?
If the owner of a folder has unlimited storage, if someone with a free account uploads a pdf to the owner's folder, will the uploaded file go onto their unlimited storage or mine?
Groups are how collaboration happens in Zotero. https://www.zotero.org/support/groups
All storage for a group library counts against the group's owner. Users with free accounts can be part of the group and upload files if they have that permission for the group. That file will only count against the group owner's storage.