That only prevents people from going to see the movie with their friends after buying their ticket. People oculd still buy tickets for their friends to go separately.
You need to tell MoviePass that you're buying a ticket for that day, and they load the money onto a debit card. You can then use that money to buy whatever ticket you want.
Because of that, MoviePass recently began requiring people to take photos of the tickets they buy. This introduced a new step to the process: now, you need to buy a ticket, take a photo for MoviePass, and then exchange it for the ticket you actually want.