It depends on the situation and what level of friend. For someone I don't know too well: "no thanks." For someone who knew me as a drinker: "No thanks, I quit drinking." That's it.
Very few things in life actually revolve around alcohol, it's just we have a distorted perspective and perceive every event as being an excuse to drink, since that's what it always was to us. For the few events or situations that are purely an excuse to drink I don't bother attending. I'm a non-drinker, why would I want to go somewhere where the sole purpose is to drink?