I know several MoviePass subscribers who do this. Apparently, if you purchase at the theatre with your MoviePass then MP counts the purchase as 'that day', but the ticket kiosk lets you select a future date and MP is none the wiser regarding the details of the purchase. People who pass by a theatre on a regular basis (on the way to work, the gym, etc) can simply collect up a batch of tickets for a future showing and then take a bunch of friends.
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.