This is a question that should be answered before anyone signs on. Some states have laws that states that tips cannot go to the house, but even in those cases I doubt a co-op situation like his would be covered.

So, given that it's a co-op and you volunteered, I'd expect that the tips would go to the house (co-op) unless something else were explicitly agreed on.

However, picking up the pizza tab for employees and not volunteers is tacky, at the very least. It should be the same for paid and unpaid workers, or if not the same, it's the unpaid workers who should get a free meal.