You know your guests better than we do. If they’re basically nice people, who might overindulge a little, other than keeping them off the road after the party, there should be no real problem. If, however, they belong to the “consume mass quantities and get rowdy, loud, and destructive” camp, I’m not sure why you’d want to have a mess to clean up and expose yourself to civil liability by fueling their behavior. Please be advised that in many states, hosts of parties where someone has had too much to drink and later caused an accident have been successfully sued!
So, you could always leave the really heavy drinkers off the guest list, and hold the barbecue inviting those who either don’t drink alcohol or drink it sensibly. Make sure you have plenty of ice-cold nonalcoholic beverages prominently and attractively available, and I’d have a wide variety: There are some outdoor cookbooks that have recipes for very festive nonalcoholic beverages such as various juices, iced tea and punches. Not every adult likes soda pop!
If you have a spouse, or a trusted mature friend, you can set up a bar, with the person playing bartender. That way it’s pretty easy to keep tabs on who is getting a little too lubricated, and diplomatically cut them off in time to let them sober up before getting behind the wheel. Water down those drinks a little with plenty of ice. You should, anyways, switch to serving nonalcoholic beverages only about an hour and a half before the projected end time of the party to allow guests to clear their heads.
Make sure that you don’t differentiate with your party beverage ware when it comes to alcoholic and nonalcoholic drinks. Being served a drink of anything should be done in the same kind of glass. Don’t make alcohol more attractive by serving it in an attractive picnic glass while you’re serving pop in a paper cup, in other words.
I’m in a similar situation with my sailboat. Every summer, I have people out for a sail, and as the skipper, it always gives me a little concern when a guest arrives with a cooler loaded with beer, or stronger. Being outdoors, in the sun and on the water, makes people drink more of whatever they’re drinking, and when that liquid happens to be alcohol, they tend to get more intoxicated than they normally would in an indoor party situation. While I do serve some alcohol on board, I’m careful to dole it out in limited amounts and interject more alcohol-free stuff. On a boat, there is always some element of risk of injury, and trust me, there is little worse than an inebriated guest heaving-ho on the deck of a sailboat in the hot sun! Unless you want to count the time where a blitzed guest fell overboard. Fortunately, she was wearing a life jacket, but I had to go into the water and tow her to where we could hoist her out with a line on the winch!
Even when I told this woman about the incident (she had little memory of it,) and how close she had come to drowning in Lake Ontario, she didn’t “get it.” Amazingly, she was very angry about the fact that I never did invite her out for a sail again.
Everyone was happy campers!