The Slack integration is available on all Hopin plans.
The Slack integration sends a message to Slack anytime someone registers for one of your events.
Follow the instructions here to get a Slack webhook.
Login to your Hopin organization, navigate to the 'Integrations' page, and click on the Slack integration:
Click on the 'Install application' button in the top right of the Slack page, and when prompted paste your Slack webhook:
You'll receive a confirmation that the webhook has been installed successfully; now register for any of the events on your organization, and you'll receive a message in Slack. You're now ready to go!