DEFINITELY tell them that they have to pay for the food. I arranged a meal for my Mum's 50th, and kind of expected everyone to realise that they would be paying (as it wasn't in my house). However, one couple (older than my M & D, so therefore of the era where if you got invited somewhere you got paid for), assumed that it was being paid for by my DAD! He was too polite to tell them that wasn't the case, and paid for them...whiuch made me really mad, as I did later explain the situation to them.
All this left me feeling awful, so do not do what I did!