If you're hosts won't be offended, take some wholemeal bread so you can have it as  HEXB choice. Stay away from cheese on burgers. You could even take your own burgers and sausages?
If you can, have lots of salads, be careful of dressings and mayonnaise. 
I find most people, when you tell them you are on SW, are pretty understanding, so if you take your own bread and stuff I'm sure they won't mind 

Try not to drink alchohol, save syns for food 

 well, I do, I find I'd rather stick to diet coke and eat lots than drink!
Hope this helps 

 And have fun! Ultimately, it's only one meal, and you can always flexi-syn!