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.