Who pays for storing the files, such as snapshots, in a group library? Is it the library owner? Or, is it the participant creating the entry?
Suppose a group wants to upgrade its storage?

The file's stored in a group library count towards the group's owner's file storage. If you want to upgrade group capacity, pay the group's owner to upgrade.
If that's not practicable, transfer ownership to a new account that's not associated with a person - and whose sole function is to serve as the owner-account of the group library.