Angels
You build the page. Their people fill it. They open it when they're ready.

Who does this
An Angel is whoever steps up. Usually it's a sister, a best friend, a husband, or the coworker who sits next to them. You don't need to be the closest person in their life. You need to be the one who's willing to set it up.
You put in their name, a photo, and a few sentences about what's going on. Then you send the link to the people who'd want to know. That's the job. The page does the rest, and the person you love doesn't have to organize any of it.
Before you talk yourself out of it
Plenty of pages are started by a coworker or the friend from college. You need to love them and be willing to set it up. That's it.
There's no spreadsheet. You put in a name, a photo, and a few sentences, and the page keeps track of who's joined and what's been said.
You send a link to a page full of notes. If someone wants to chip in, there's a button for that. You don't have to bring it up, and neither do they.
Step by step

Their name, a photo, and a few sentences about what's going on, in your own words. If they're under 18, you'll name a guardian. It takes about ten minutes, and until they open it, the page never shows their diagnosis to anyone you didn't tell.

Start with five people who'd drop everything for them. Text it, don't blast it. The page isn't listed anywhere, so the link is the only way in, and the first few notes set the tone for everyone who comes after.

People write what they'd say in person. Someone posts a photo from 2009. Someone chips in by card, or doesn't. When there's news, you post an update to everyone, or just to family.

When it feels full enough, you invite them. They walk into all of it at once and decide what happens next. After that you keep the updates coming, and the notes and photos can become a short film they get to keep.
A name and an email to sign up. About ten minutes to build the page. They won't see it until you're ready.