First of all, let me congratulate you on having a great social life. That is something to be happy about.
There is a lot of great, healthy stuff you can eat at a barbecue. Any of the meats cooked on the grill would be alright. Do they serve salad, corn on the cob, grilled vegetables?
If they don't, maybe you can bring some along as your contribution to the meal. Bring a big leafy salad or a fruit salad. Bring some veggies on skewers to toss on the grill.
Now I'm hungry.