I write stuff down like mad: PC names and a few words to sketch them out, key NPCs they are attached to, anybody Vx mentions by name and how they connect. It's not so much a web as a little booklet with each PC at the top of a page. When Keeler got a gang, I went through and pulled some names from the already named NPCs, then rounded out. So I had 15 names with a word or two next to them, like this:
Peaches - younger male, drives the caddy, has a crush on Rose
Mercer - woman, good w/machines, rides with Peaches
Last and Exit - older men, lovers, rear-guard in the Pick Up. Last drives, Exit runs the mounted machine gun.
etc.
This way, when we get into trouble, I know who's hurt (Exit, Mice) and who's dead (Last, Mimi, Touch), and I can use that in fiction: "Sorry Rose, I can't be three places at once, I'm down 5 guys, right?! I can handle the market and leave you a couple folks, but I can't sit on Drain, too. You'll have to ask Do or somebody."