SEB'S Dex was obviously here because he lives here - he didn't really drink much and ended up being mostly sober for the night because he figured Jack would mostly be spending time with Bitty as their first NYE as a ~married couple and kept the basement Very Locked so nobody could get into his room, probably played some games with people and vanished some time around midnight with a shirtless Nursey in tow.
Yuri was probably here purely because Otabek was and ends up taking advantage of the free drinks which definitely loosens him up and makes him less of an asshole. Probably got into a dance off with somebody at some point.
Zagreus also ended up here by accident because he doesn't know what New Years is but please teach him ALL of the games. He wouldn't really get very drunk from the alcohol but is a cheerful guy anyway and will just hang out with anybody. If requested, he will carry you around.