Type something in the note, then tap the three-dots icon in the top-right corner of the page, and tap Share note.

Go back one page and select how you’d like to share the note with someone else.

To make it easy to differentiate between your messages and those by your contact, tap the three-dots icon in the top-right corner and select Manage Shared Note.

Then tap the three-dots icon in the top-right corner, go to Share Options, and change the document permission to View only.

While Apple Notes (or for that matter, Google Docs) allows you to have collaborative chats, these aren’t really the apps you want to turn to for true privacy.

Once you’ve started a chat in Signal, tap the contact’s name at the top of the page and select Disappearing Messages.