How to structure Firebase data and rules for sharing data between users

FirebaseShareData

Since the user decides who'd they like to share the list with, I'd store that information in the user data itself. For example:

{
  "rules": {
    "$userPath": {
      ".write": "$userPath == 'acc_' + auth.id",
      ".read": "$userPath == 'acc_' + auth.id || root.child($userPath).child('shared').hasChild(auth.id)"
    }
  }
}

And then store the list of users the data is shared with in acc_userid/shared/