a) when upload is done in any folder
b) to every user that is assigned to that folder
?
Sounds like a rule which is hard to define in the UI, so I think I should add possibility for defining custom functional rules, meaning there could plugin script for determining what to do. It could be applied to
a) which events apply (script could have a function that return TRUE if it applies, in your case it would apply only for upload events, perhaps with some logic for which folders)
b) which users are mail recipients (script could return list of users based on the fired event, in your case it would just retrieve list of users for the folder)
and so on.
Perhaps at some point, I'll open issue for this.