To email multiple registrants within a particular camp or multiple camps, go to:
Communication (speech bubble icon on the left-blue bar)
Click “Contacts”
Click the camp/s whose registrants you would like to email on the left
Go to the bottom of the page and click “Email all ### Registrants” on the bottom right
From the Send Email pop-up, you can draft an email to all of your registrants at once, then select “Send” in the bottom right