Currently when a calendar is modified, you can send notification emails to the owner of the calendar.
You also can send notification emails, when the owner makes changes, to a list of email addresses.
This is OK, if only 2 people need to make changes or be informed.
But when there are more than 2, there is no way to get informed about all changes, not made by yourself or the owner.

Therefore we want an additional calendar option:
"When someone else modifies my calendar, send a mail to ..."

With that all users can get an email upon modifications in this calendar, independent if made by the owner (through other option) or someone else (this option).