Updating since it’s been a while. Currently Square Cash does not have an API that we can use to integrate. At this point we would need Square Cash to change in order for us to do the integration.Nathan Payne commented
Please integrate Square Cash—this is the method me and most of my friends use for payment and Splitwise is way less useful without it. I'd use Venmo, but PayPal (the owner of Venmo) doesn't have the greatest reputation for consumer and seller protection so I prefer to not use their products if at all possible.