Do my users have to register with google to use Revenuecat?

Yes you can, only requirement of an Id is to be unique

You can use your own ID from your system as long as it’s unique like @Emre said.

Here’s the spot in the docs that mentions using your own ID https://www.revenuecat.com/docs/user-ids#provide-app-user-id-on-configuration

To answer your question “Do my users have to register with google to use RevenueCat?”: There is no requirement for your users to register with Google to use RevenueCat. RevenueCat is a service that ties to your Google or Apple developer account. The users don’t necessarily have to be aware that you are using RevenueCat