I would not tell people they had to bring food on the invitation. That feels to me like im charging admission to my party. Thst said I do hope people will call or text me and ask, because some of my friends are amazing cooks, and thr ones that aren’t have good taste in wine.
I would have preferred if they didn’t bring anything because everything was provided for, but if they had wanted to bring anything, I would had them bring something that was consumable at the barbecue.
Call and ask. When I’m entertaining a group of people, I plan the menu and serving dishes and so forth carefully. I really don’t want someone’s random dish on a plastic plate. If people ask what they can bring, I’ll usually say “something to drink, if you’d like.” That leaves it up to them whether to bring wine, beer, soft drinks, juice…people usually bring what they like to drink, and extra bottles of wine never go amiss.
Normally, if someone is barbecuing and would like side dishes, they will say so on the invitation. Quite common for this sort of casual event. Or, in the case of an alumni welcoming event, the alums will have arranged contributions between themselves ahead of time.