Easist way to email registrants

What is the best way to send an email to all who have registered for a conference through Jotfor?

Unfortunately, Jotform does not allow to send mass emails to your users. However, you may alternatively download an Excel report to gather the email addresses from your submissions: https://www.jotform.com/help/101-How-to-Create-an-Excel-Report 

I hope this helps.