I attend/host/plan alot of potlucks with my family and social circle and we don’t have any hard and fast rules. Usually, whomever is hosting is the one to make the rice/biryani/pulao because let’s face it, that is best served hot right from the degchi lol!!!
As far as leftovers go, we usually just divvy up amongst ourselves, and whatever is left in your dish/bartan, you take it back home with you!!