NYMets, you are right that they will have drinks and not notice you aren't. If they do and pressure you, even if you leave, they'll move on and it won't be a big deal. I think as drinkers we get this illusion they are having a blast, picking on us the whole time(nerd!) , and reality is once you're gone, they move on to wherever the drink takes them next, or event, etc.
Then the next day the "You missed out Man!" Crap might start, again another illusion. I missed out making a butt of myself, or watching u make a butt of yourself, etc. They look and feel good? I remember faking how awesome and fine I was after a party night, another illusion.
I like how you are setting up expectations and exit plans. When the worry creeps in about peer pressure, just know the moment is fleeting. Easier said than done.
I absolutely have girlfriends who like to have a tipsy girl night, some pressure but the adult in them takes over. Early 20s is harder, but by 30s while still younger (i vote it is) people have seen addiction or have it and the pressure lifts.
I'm bogged down with school, and so posting less than I'd like, but I wanted to wish you a good time.