I think that the 'A' word is scary (atkins - eek!). I think that people hear it, and immediately it conjures up pictures of bacon bingin', chomping down on a beef carcaus, sorts of images. I think if you just say 'low carb', it might soften the blow a bit. Just explain that you were feeling really tired and having low energy before, and wanted to try cutting out the carbs and sugar. I think people respond better when it is a 'feel better' thing, and not a weight loss thing.
I feel for you about your dinner though - sounds rough. Just fill up on things you are allowed, and do the best you can with the rest. And if you are bringing dessert, bring the cheesecake!! It is so rich and indulgent, and doesn't taste at all sugar free if you do it up right. Good luck!!