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:
Step 1: Navigate to Email section of the platform
In your event's left navigation menu locate Email and from there select New Email
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.
Step 4: Send!