When you first start Weight Watchers, it is your decision if you want to share that with anyone. Some people do not because they are embarrassed, or are afraid of failing, or don't want people watching their every move.
If you do let people know, be prepared for questions and comments. The one question that always made me crazy is when someone would say to me:
"Can you eat that?"
This bothers me because it's none of anyone's business what I'm (or you're) eating. But I've since realized that people who have never been on WW just do not understand how it works. They are used to restrictive (and unrealistic) diets. So I take the opportunity to educate them-
"Yes. I can eat whatever I want. However, I may or may not choose to eat that."
First of all - erase the word "can't" from your vocabulary - not just with Weight Watchers, but in general. There is nothing you can't eat. There is nothing you can't do. Can't is a horrible word. Any time you hear yourself say to someone "I can't eat that, I'm on Weight Watchers", catch yourself and change it in your head so the next time it comes out more like "I do not want that, thank you."
This is especially helpful in situations where people try to push food on you. Sometimes innocently, sometimes maliciously. How you reply to them speaks volumes. Your stating "I can't eat that" implies that you want to but are 'not allowed'. This gives the food pusher more power in the situation to tempt you further. Be firm (even if you are tempted) and assertive, and reply that you do not want it. Stop saying you cannot have it. And the more assertively you reply makes it clear they should not ask again.
It's really an attitude adjustment in how you view what you are doing. Start practicing how you answer that question in your head until it comes as second nature out loud. The more you say it, the more it will sink in for yourself, and the more people will understand that you mean it. You will be more confident and that confidence will be apparent to those around you.