When you are creating or editing a class (workshop, event, etc.) in Step 5 - Filters & Settings you are given the option to have an automatic email sent out to all attendees who registered for that event 1 day before the start date. Be sure to have this function checked if you want each attendee to receive the reminder email. Usually by default this option is enabled, which is recommended, but you have the option of disabling it.
Each attendee automatically receives an email when they register to an event, or if their registration is cancelled, or if they receive any refunds.
If you would like to message your registered participants for any other reason related to the class or event they are registered for, you can use the memo feature. Click on Attendees within the class of interest. In the top right hand area you will see an arrow navigation menu. Click on that and you will see the option to Send a memo to attendees. Follow through by sending your message or announcement to the entire group of registered participants or your choice of selected ones.