I've actually worried before about not drinking being a social handicap at today's hip tech startups. It certainly makes bonding with the rest of your team more difficult if their primary social venue is the bar.
I would be honest with them but if you're that concerned about "fitting in"...just order a soda. If somebody asks, tell them you prefer mixed drinks and that it's a rum and coke. Nobody will know the difference.
Or that you're driving/riding, or that you're in training for a Marathon... There are loads of socially acceptable answers if you don't feel comfortable saying "I don't drink" (not that there's anything wrong with that).
