The non-G-suite user simply has to request a pincode from within the email invite they get. That would issue them with a code that they can use to unlock the G Suite document, sheet or slide that they need to access.

Since it's still just a beta release, not everyone will be able to enjoy the feature. Organizations would need to sign up for it, since Google has to whitelist a non-G-suite domain for the feature to work. The company will most likely have to check the domain and make sure it's legit, as well, which is why Google doesn't guarantee that everyone that signs up will be able to participate. Those teaming up with classmates and colleagues who don't have Google accounts will have to convince them to sign up for now.