Here you can see what options are enabled and how many logins there have been. Two checkboxes are important for notifications.
"Send Email Notifications to Viewer" This will send an email notification every time a new file is added to the shared folder or comments are made on the share links.
"Send Email Notifications to Viewer" Deselect this, and the sender will no receive any notification emails.
There is also a master option under the Preferences menu to set the default setting for notifications for the entire account. If the default is set to "off", it is still possible to enable notifications for specific share users. Remember, you need to be an administrator to set the master setting.