Events come with a discussion board that you can use to easily keep in touch with your event attendees. For example, you could post some extra instructions in the discussion board, or your attendees could ask you questions like directions to the event and you could answer them.
Access the discussion board by clicking on the "Discussion Board" tab on your manage page. Your attendees will see an identical "Discussion Board" tab on their thank you page after they RSVP.
To post, simply write a note in the comment text box and click "Post To The Discussion Board." This will send an email to all attendees with what you've posted and inviting them to respond. Attendees can post from their thank you page, which will spawn an email to you and other attendees.
Comments will appear in chronological order, with the newest comments at the top. To respond to a specific comment, click the "Reply" link under that comment.
Also note that this discussion board is only semi-private. Only the event creator and people who have RSVPd will have access, but anyone can RSVP for your event by entering their email and zip/postal code.
You can turn off the discussion board if you'd like as well, by using the controls in the "Discussion Board" tab on your event's manage page. If you turn off the discussion board, your attendees will have no way to contact you if you haven't filled in the public host information when creating or editing your event.