In the Users tab:
select target user(s) / user group(s) and right-click on any of the selected items;
choose "Change notification settings" and the frequency in the drop-down menu;
click OK in the newly appeared pop-up to confirm the change.
The option to change notification settings is also available under more options menu next to the name of a user group.
Notifications will be sent out only in case there were any uploads made in the given timeframe and only with reference to the documents available to the specified users.
Additionally, users can change the frequency of their own notifications in My notifications settings.