Hi Firas BD,
you can achieve this by modifying CalLocalServiceImpl's remindUser() method using ext plugin.
If you want to send a notification to a specific group users then you just have to modify remindUser method. By default it just sends a notification to owner.
You can get the users of specific group by UserLocalServiceUtil.getGroupUsers(groupID) method, than iterate list of users in remindUser method to send a notification.
Firmi prego dentro per inbandierare questo come inadeguato.