you poor thing. What rude friends to have put you in this situation and not even offer to contibute and to seem ungrateful.

I think you need to say to them "RIght we need to go grocery shopping, do you want to give me some money and I'll get it or do you want to come with and we can go halves?"

Even when we stay we family, we always pay for our own meals if we go out to a restaurant and will often shout them as a thankyou. Alot of times they dont accept but it is nice to be offer.

hope you can find a solution.