Login to your WP-admin (WP dashboard) and go to Event Espresso --> Events. Then hover over the name of your event and then click on Registrations. This will take you to the registrations overview page that shows attendees/registrants for a specific event.
Look in the top right corner of the screen and click on Screen Options. Next, look for the Registrations field and change it to a larger value such as 75 or 100 and then click Apply which will save changes and the page will refresh.
You can now select all registrants by placing a checkmark in the heading column which will select all registrants on the page or you can select only certain guests. Once you have selected your registrants, scroll down towards the end of the page and click on the Send Batch Message button. A small window will appear and you can select a template such as the Global option and then enter a message and then click on the Send button to send out your message.
The best way to learn is to take a look at the video below and then try it out on your own. Don't worry about setting up something wrong and if you are not sure, then reach out via the Event Espresso support forums.
##Send a message to your attendees/registrants
Click the play button on the video below:
References:
https://eventespresso.com/topic/ee4-reminder-email-to-those-whom-have-registered/