Bring a bunch of friends without asking
Not cool. Bringing five friends without checking with your host throws off the entire ratio. Now someone’s gotta run out and get more patties, buns, ice, beer -- everything. Also, there’s probably a reason they weren’t invited in the first place.
Not help clean up/wait to be asked to clean up
Your host went to the trouble of organizing this barbecue, plus buying and cooking all the provisions -- the least you can do is help them clean up as they go.
Leave without saying goodbye
This isn’t a bar. Your friends threw an awesome barbecue and fed you. Say thanks.