I like Joplin, and the sharing is done via Dropbox or a number of other backends. Have a Dropbox shared folder that everyone has access to, and you can edit notes. Joplin is a great note taker in general, as well, and the mobile app is great.

Joplin isn't that easy to setup for family. It also doesn't work like that for Dropbox, it links to a specific app folder on your own account. I'm also not sure about distributed editing it would result in a lot of conflict notes that you manually must fix.

Just use Notion or Google Drive folder with docs if you don't have too many notes.

Assuming you're well aware of something like a shared Google Doc and you need something a little more fully featured...

For "easily shared" it's hard to beat Notion. They have a really good mobile experience and the ability to control permissions and sharing at a detailed level.

The only drawback with that recommendation is you also asked for "very simple," which Notion is definitely not. But, you don't have to use all the features available--you can use the complicated tool in a simple way if you like.

for Apple ecosystem, the Notes application works well for "simple" shared notes