Once you board the Splendor, you can go to the Lido deck for the buffet, or the rotisserie. The rotisserie was very quiet, so I'd arrange to meet them up there. It will be much easier to find other people.
You've booked the cruise yet do not know the itinerary? Perhaps I am misreading that. Do you mean ports, or what you do once you board? The boarding process was a cluster. I would arrange for parents to get there early (or everyone else) and you later (or parents later). You are bound to see one another outside waiting to board unless you get there hours apart.