Hawkes

So a few things I learned during our cruise on Harmony this past summer.  Each guest can download and install the RCCL app and if I remember correctly once everyone is signed up and connected to their reservation number you can use that as a means to communicate even without having an internet package.  We did not do this ourselves as we had bought a four device internet package.  We just put everyone's phones into Airplane mode, turned on WiFi calling on them and were up and running.  The only phone that had an issue was mine since I was the only Android user.  Texting between iPhones and Android does not work.  Period.  All iPhones users were able to text between each other with no problem.  WiFi calling worked great as well especially to get back in touch with our realtor back home who was in the process of selling our house while we were away.  This allowed me to avoid roaming charges on the cell network and still be reachable.
What I can't say is if anything has changed since July when we went and I also can't say if it allows you to communicate with other reservations that are not connected to yours.  We had three cabins for the six of us and the kids two cabins were connected to our main cabin and for the kids that I setup and registered the app with, I could see them, but never really used it because of the other options I mentioned above.  The RCCL app is a great tool though and I suggest you install it and get it set up before getting on the ship because once you are on the ship and they are broadcasting to everyone to install it, it will take forever to download.
One other note about the internet is you decide to get it.  You will get one passcode that works for as many devices as you pay for unless you buy a passcode separately for each person (more expensive).  Once you connect the max number of devices to it that you paid for, no more devices can use that code UNLESS you log one of them off from the system.  It's a specific URL to go to on the browser of your phone to logoff.  Once you do that, a different device can connect.  We did this all week as necessary.  It allowed the four kids to share two of the four devices we paid for.  I think it enhanced their experience as they were able to do what they love to do... "snap" their friends and post to their stories and share their experiences in close to real-time.  The other two device connections we paid for went to my wife and I so we could stay in touch regarding the house and since I'm in IT I was still "on call" for my clients.
Have a great time!
-Hawkes