I haven't used the app on a ship since the recent app upgrade, but I suspect the on-board functionality hasn't changed. When you board, each passenger will enter their Name/Birthday/Stateroom information. From there you can set up the on-board chat between all of the different passengers in your group.
With the new app, prior to your cruise, you can see information about your reservation. Each passenger can create/use their own Disney account. If the reservation isn't associated with their account then the reservation number and passenger birthday will populate the account with the reservation.