I have never been able to message guests of an event that I have created through one of the brands that I am an admin for and today I noticed someone else was able to... I did some research to find that if you create an event though your personal profile page, you can message the people who RSVP yes, no, maybe and even those who haven't replied. You currently cannot use this function through your brand page.
I just wanted to share....At this time, it is not possible to message all the fans attending an event you have created on your Page. You will need to use the Page’s "Updates" feature in order to send out a message to your fans. /help/?faq=15206
To message the event guest list FROM YOUR PERSONAL PROFILE PAGE, go to the event’s main page and click "Message All Guests" beneath the event profile photo. You will be able to send a message to all guests, or to different groups of guests based on RSVP status.
Please note that you can only message all guests if the total number of guests who are attending, maybe attending, or who have not yet replied does not exceed 5,000. Also, if an event is hosted by a Page, the Page admin will not see the option to send a message to event guests. Individuals may be added as event admins in order to have this option. -- I haven't seen this option though.
+Shana.