When signed in as an admin, you can send an email to everyone who's registered for a form by following these instructions:
Go to Registration
Click Participants in the upper right OR click the number of participants under a form
Use the checkboxes to select the number of participants you want to message
Use 'Message Participants' under the Actions button
Craft and send your message!
Alternatively:
Go to Organization > Reports, then under filters click choose Registration as the type
Choose the Organization and form you want to email participants of
Go to the bottom of the table and change it from Show 25 entries to Show 250 entries to email them all at once
In the top right click the email icon and compose your message
For a full overview of all messaging options, click here