In Nothing Fancy, there are a couple of pages devoted to a "DIY Martini Bar". I liked the idea and the recipe was legit, so I made it for an Alison Roman-themed cookbook club gathering.
The recipe is a 50:50 gin vermouth ratio (similar to Audrey Saunders' Fitty Fitty Martini, minus the orange bitters), pre-diluted with 20% water, so it's on the lighter side. You keep it chilled, and your guests can pour themselves a glass on ice and add their favorite garnish(es): I had lemon peel, two types of olives (feta-stuffed divinas which are super umami, and, on the other end of the olive spectrum, castelvetranos which taste green and almost sweet, like green almonds), and home-brined onions for Gibsons (my current go-to drink). I used Tanqueray 10 and Noilly Prat dry vermouth (I used the extra dry because it is milder/less busy than the dry, which works best in a 50:50). I made a 1/2 recipe which yielded a full bottle. There were some leftovers that I was happy to bring back home. 
