Tight bastards, you could try talk to them but you are probably wasting your time with people this tight and cheeky. I'd be as bold as them
"Mary can you peel an orange in your pocket too? Don't be tight, we can see the sweets, take them out, offer one and then you'll have more enjoyment eating them openly rather than faux yawning and popping them in to your mouth when you think nobody is looking"
"No thank you I don't want coke, but you might offer it around, the food in the fridge is for everyone, in this house it's not what's yours is mine and what's mine is mine alone"
"Girls bring snacks by all means, but you aren't 8, don't sit there and miserly hide it, it's weird and rude. If you don't want to share them keep it and eat it in your room"
There's a lot of projecting on this thread, my mil starves us with her tiny portions so I bring snacks but they are to share - eh thanks for sharing your story, but it's totally different. These people are eating and drinking and being paid for when out and then acting like golum over a fucking packet of doritos, it's rude!