No idea - when we do these type of events it's kinda a "Me and my mate are doing this, whoever shows up is welcome". So I'm guessing Shell and a few others have already agreed on eating dinner then watching a movie and then gave an open invitation to the rest of us. So it's not like a "Put your name down here - Fees here and you must come if paid" etc.
Free event (Pay for dinner / Movie ticket). Shell might ring a dinner place to book a table for at least x amount of people or might just be a wing-it thing when we rock up (Let's eat here, it looks like it'll cater for us).
Hope this helps.