Our organisation (ad agency, 200+ staff) works for 70+ customers. The customers are assigend to teams. In Box, every customer has a project folder, and a team is assigned to a folder (with editor rights).
But not all members of a team work for all clients of a team, and even if the work for the respective customer, they do not participate at all folder/files of a customer. Regards notification this means that all team members are spammed with notifications for clients they don't work for and files or folder they never participated on.

I would mean a huge admin effort for the folder co-owners if we would change the way of assigning users to a folder (currently it's done on group per folder-level, the problem wouldn't come up if every single user is manually assigned to every sinlge file/folder)

An idea would be that a user could get a "unsubscribe" function on a folder and file level. This would mean, they wouldn' receive notifications on the unsubscribed folders/files.