I work at a Starbucks. The scanners are usually customer facing and all the barista needs to know is that you are going to pay with an app on your phone. As long as there are no complications I doubt the average barista will know or care exactly how you paid. As for the crowd sourced gold card, Starbucks' loyalty program gives you a special card after 30 transactions (Gold Card), once you hit that level, you get a free drink or food item. Every subsequent 12 transactions earns another free drink or food item. These would obviously be earned very quickly if multiple people are using the same account. However, I feel that if this is the way this app works, that it opens itself up to both Starbucks shutting it down quickly and user exploits. You could just generate your own image of this accounts barcode and use it whenever you'd like. The worst that would happen is that the barista would inform you that there wasn't enough money on the card, or that it was an invalid account, something that happens often. So I doubt this is the case with this app, I am interested in how they are running this though.
Edit: Up above it is mentioned that each customer gets their own account. So this isn't an issue.