J.B.
In my circle of friends, the expectation is that you bring something. People shouldn't come to dinner empty handed, so better something that can contribute to the meal vs. some meaningless hostess gift. I don't ask people to bring things, but everyone offers to and I always take them up on it - things like a salad, a bottle of wine, or dessert. I tend to know who likes to make or bring what. For example, I am the dessert lady so everyone suggests dessert when I offer. I have a friend who is great at appetizers, one who likes to throw together beautiful salads, one who loves to bring desserts from her favorite bakery, and one who is great at picking out wine. It's not a potluck - whoever hosts provides the main meal, beverages, appetizers etc. but it's nice to welcome extra dishes from those who offer to provide them.