Ticketed events come with a discussion board that you can use to easily keep in touch with your ticket purchasers. For example, you could post some extra instructions in the discussion board, or your purchasers 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 ticket purchasers will see an identical "Discussion Board" tab on their thank you page after they buy tickets.
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 purchasers with what you've posted and inviting them to respond. Purchasers can post from their thank you page, which will spawn an email to you and other purchasers.
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 ticketed event creator and people who have purchased tickets will have access, but anyone can purchase tickets for your event.
You can turn off the discussion board if you'd like as well, by using the controls in the "Discussion Board" tab on your ticketed event's manage page. If you turn off the discussion board, your purchasers will have no way to contact you if you haven't filled in the public host information when creating or editing your ticketed event.