While the app is free to use and anyone can freely receive invites, one caveat is that you can only send invites if you have a paid iCloud+ account. This is a bummer, but it makes

Apple Invites allows iPhone users with an iCloud+ subscription to create and share customized event invites and integrate photos from their library.

Add another one to Apple's list of third-party apps that it has endangered, as Apple Invites threatens the livelihood of event planning app Partiful.