When you did the 30 days, how did you deal with the social group? Could some of the same tactics work again?
I know I haven't told any of my friends that I've stopped drinking either. Definitely adds to the challenge of staying sober when people are offering you drinks. I've had success with "I just don't feel like it" and "I have work to do later."
A trip to Amsterdam with drinking buddies seems like a definite no-go. Given it is 15 people, I'm assuming they have a date picked out. Why not just say you're visiting your parents that weekend and can't do Amsterdam? Maybe just say that you're saving up money for something and don't want to spend on a big weekend?
Also, from an outsider's perspective, it sounds like normal peer pressure to try to get you to go by saying they're not going if you don't. It's 15 people.....does it really all hinge on you being there? doesn't seem logical.