If its an important party like my engagement or birthday or something we supply majority of the stuff like standard meat and a few drinks like wine, softdrink and beer.
If its just a small BBQ with friends I would supply salad and bread etc but usually they bring their meat and alcohol. I always have spare softdrink for guests.
When we go to BBQ with friends we always bring our own drinks and usually meat unless they say not to. We are happy to do pay for own, we can't afford to pay for others usually so we don't expect others to do so for us.