Email addresses for all registered athletes are available when you "Export as CSV" from the Registrants tab of your competition.
While managing your competition:
Click the “Registrants” tab under the “Management” subsection. From this view, your various divisions will populate the subsections.
Select any division from the list shown (all divisions are exported in one CSV export operation, so it does not matter which division you select in this step).
Click "Export as CSV" at the upper right (circled in red above) and select a location to save your comma separated value spreadsheet of user registration information.
Open the saved CSV using your favorite spreadsheet software and note email addresses for your athlete registrants are listed in the "athlete_email" column (see the example below, where athlete "Demo Athlete" has listed their email address as email@example.com).
You can then use your favorite email client to email your athletes.