General event messages can be sent using the 'Message' button in the Schedule or Attendees toolbar. If you are managing an Event with a schedule, you will find the Message button in the Schedule and in the Volunteers tab. If you're organizing a Gathering, it will in your Volunteers tab.
The Message button is for all of those little things that come up as you welcome new signups and manage your event. The message template includes basic information about the event along with your personal message.
When sent, it looks something like this: