Don't do complicated breakfasts, toast, brown bread, jam, honey, and peanut butter will be fine. Forget overnight oats, not everyone likes them.
Lunches ~~ Make soup, mixed veg and bean or a minestrone style or potato and leek all are cheap to make. A thick soup served with bread will be filling.. Use vegetable stock granules instead of chicken stock! Make the soup ahead of time if possible.
Other lunch ideas ~~ hummus & pitta bread or Tzadziki is another spread you could make it is plain greek yoghurt with grated cucumber and chopped mint stirred in.
Cheeses, hard boiled eggs, tomatoes, radishes and cucumber and other salad items with bread for them to help themselves to, though be wary that the first comers don't eat everything. You can be generous with the eggs, (cheep cheep!) but miserly with the expensive cheese.
Dinner ideas ~~ pasta Primavera, make this with pasta shapes such a swirls, twists, or bows. Cook a good amount of small green vegetables, sliced courgette, broccoli florets, chopped green beans, peas, chopped spring onions, frozen spinach leaves....Meanwhile cook then drain the pasta. Tip the vegetables into the pasta and stir well to distribute. Serve with grated cheese to hand round.
Make a thick vegetable stew, carrots, potatoes, parsnips, turnips, cabbage, celery, onions & garlic. Use vegetable stock for flavour, and a little rice to pad it out. Serve with a choice of instant noodles (buy cheap ones) and cous-cous.
For puddings ~~ Yoghurt and stewed fruit (what ever is cheaper).
Deconstructed cheese cake, break ginger biscuits into crumbs and place in small glasses, add a 50/50 mixture of yoghurt and plain cream cheese with a little sugar, top each glass with chopped summer fruit or stewed fruit.
One thing I have found with large groups like that, is that they can be very greedy. Those earliest to the table will eat two or three portions leaving others without, so where ever possible portion out the individual servings yourself.
Get the group themselves to help you with the food prep, before they go out in the mornings for instance.