When it comes time to pay up, you can use Google Wallet to both send and receive funds. If you're the plan manager, you can set up automatic transfers so that any repayments go directly to your bank account.

The feature is available in the billing section of your Fi account. It probably won't convince you to switch to Fi by itself, since there are still plenty of other hurdles (needing everyone to use compatible phones, for a start). However, it could be a tipping point if you were already thinking of signing up, whether you're starting from scratch or joining someone else's plan. Instead of breaking out the calculator every month, you could let Google do the hard work and spend more time actually using your service.