What about GROUP SHARED NOTES ?
When we go on group vacation, I like to start a group Iphone shared Note.
This note could hold a rough Daily Agenda. it could mention some restaurants that we might eat at, or something like the different flights & times.
It's a great way to share the vacation planning load & also gives people an easy place to follow along so that they aren't 100% dependent on 1 person.
It's a good way to avoid arguments

or for people to know what to wear for tomorrow. ▶ "The info is in the Group Note"
And since things don't always go to plan, it is 100% editable by anyone with Read/Write Access........so anyone can move things around & everyone can get a notification & see the changes.
The above is great IF....everyone has an iphone.
What I've found is that if I try to add or share the Note with an Android user, the Android user only gets the info in static like text while the Iphone users can still make edits/updates.
Do any of the other apps mentioned in this thread also include the SHARED NOTE functionality ?
I do use WhatsApp...........and I don't think they include this (i wish that they did)