theashleyb

If several members of your group intend on paying for Internet service on board the ship, then there are several apps like "whatsapp" that allow you text to phones over the wifi network rather than the cellular network.

Tried Walkie Talkies once or twice, usually ended up getting not enough signal or too many people on the channel to be effective but got a little use out of them.
 
Tried the rental iPhones on Allure, waste of money, too many dead zones that you had no connection in.
 
We do a lot of "we will be at x place around y time, see you there or if not find you somewhere"

I tried the walkie-talkie path once. It proved less than effective. I have heard others say they used walkie-talkies and found them useful. I didn't...but maybe I just had bad luck or old technology (probably the latter).

Some folks get a whiteboard and post it on their door.
 
If you're on a Quantum class ship, you can use the Royal iQ app and it has built in messaging.
 
If you get the internet package, you can use a messaging app.