Sending followup emails to those who haven't responded to your event yet can be done from the Email section of the platform. Here are the steps:
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​Step 1: Navigate to Email section of the platform

In your event's left navigation menu locate Email Communications and from there select New Email
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Step 2: Compose the email

Step 3: Select recipients
In this step you will be able to select 'Invitees who haven't replied' as the audience for the email.
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Step 4: Send!