Get a drink. Go to the bathroom and pour it out. Fill it with water. Voila! Nurse that drink all night long. Nobody will be able to tell you're not drinking at all.

Alternatively tell them you're on meds and can't drink. If they dig, tell them it's antibiotics for an ear infection.

Yeah, it sucks having friends who must get drunk to have fun. It sucks when you're 35 and they STILL haven't grown out of it. (One reason I'm no longer friends with these people----a long line of alcoholism in my family makes me not want to touch the stuff, plus bad mixing with meds, plus it skyrockets my anxiety. Any one reason is reason enough to not drink.)