From my personal experience: The places that I've seen so far that use that system (including one place that I regularly attend with a larger group) do not require an account or anything: Scan a QR code, if you don't want to log in just enter your credit card number, and start ordering. If you're not comfortable with that approach there's usually also some way to order without a phone.
Regarding "scam their friends": 99% of the time it's not about "scamming": Cashiers ask people for which items they want to pay and mistakes happen because people sometimes remember things wrong. Once everyone paid there are still some few unaccounted items for the table due to those honest mistakes.