You can make your event an invite-only one by creating a password for people to access it. On the first page when setting up your event, also reached via the “General” tab within “Settings”, you can add a password for your event.
Enter whatever password you’d like to use here and Save the page.
Give people the url to your event as normal, or link to it from your website. When people go there, they’ll first be prompted to enter the password you’ve set.
Once they enter the correct password, they’ll be taken to the form and can register for your event.
This isn’t a feature you’ll use often, but sometimes it’ll be exactly what you want.