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The roll call for your sailing can be found on Cruise Critic website- click on 'Boards', then Roll Calls and you can find out some of the pax you will be sailing with and ask questions. It's just a forum to meet people prior to cruising.... nothing to do with the passenger manifest.
If enough people sign up for a Meet and Mingle (normally 25) RCCL will host a get together with free snacks etc. for you.
The roll call is the list of passengers on the ship and it is used to check taht all are accounted for in the event of an emergency like fire drills et.
Incorrect answer by Mr/Mrs McDermott - the roll call is a gathering organised by RCI - it is called a Meet and Mingle, You register for it on the web. It is a way to meet and chat with fellow passengers before you board and perhaps share shore excursions. RCI will host a party on board if the roll call exceeds 25 people.