To be able to join, you'll have to already be using the Gmail app, have a non-Gmail address to add and have iOS 10 or later installed on your phone. Take note that Google has to share whatever info you give it with Apple, since this is a TestFlight program. You can sign up through the beta testing page Google set up for the feature: if you get accepted, you'll receive instructions from Apple on how to proceed. There's no telling when and whether the feature will ever be widely available, so this could be your only chance to take it for a spin.