Churches and other civic groups have potlucks too. In my experience-basically middle class American-large summer backyard barbecues are often like this, with the host buying burgers and hot dogs for everyone, but families all contribute food and maybe byob as well.

The host might even provide the main dish, and assign categories-bring a salad or bring a dessert. At a meal with more participants you might bring not enough for everyone, but more than just you-like a full sized 10x14 inch baking pan. In the US a pot-luck is a meal where everyone is invited to bring a dish to share, in an amount to feed the group.