Email addresses for all registered athletes are available when you export all athlete info 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 "Request an Export" at the upper right.
Check your email - the CSV download will be sent to the email address associated to your Strongest account.
From your email - 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 firstname.lastname@example.org).
You can then use your favorite email client to email your athletes.