SmarterMail includes a Shared Resource feature that I believe you would benefit from. Rather than sharing single events among your drivers' calendars, you can share out your main customer calendar to your drivers. You have the option of giving them full control or making it read-only. Once they have attached the calendar to their account, they'll be able to toggle between their personal calendar and the main customer calendar. When creating events, you can enter the driver as an attendee and they'll receive an email notification when the event is created.
Below are two links that will give you more information on shared resources. This involves you sharing the resource and your user's attaching the resource.
Let me know if you have any questions!