When you create an event or want to share an event from your calendar on Facebook or via email you should use a direct link from you calendar event. From the Dashboard > Events > Edit (the event), make sure you use the Open Event Page type for the User Interaction for event click:
After you click on the Open Event Page, you will get a link specific to this event that you can then use on Facebook and in emails that you send. This link allows users to open the event directly and comes in very handy if you are collecting RSVPs.
On this screen you can also make sure you have your RSVPs turned on and manage how they are handled: