contact the special services department, they might have some suggestions, even cruise line specific items, also if you want to reserve a room to meet and greet you would have to do it thru that department.
Useful items would be those water proof containers to store money and keys while snorkling. Mini bottles of sunscreen. Baseball caps or sun visors. Disposable camera for when people run out of memory or film. Can cooler, or plastic water bottle. A book of cross word puzzles for the plane ride or lazy days at sea. A mini address book for keeping in touch with new cruise friends. Post it notes and a pen for leaving notes on cabin doors or for the room steward. Tiny bottle of baby powder or corn starch, for hot humid days, to stay dry. Dollar store rain ponchos for shore excursions just in case it rains.
I have two suggestions for where to meet when you set sail. If you want a small degree of quiet so you can make plans, meet on Deck 4 in the atrium, it is easy to find and has plenty of seating. However if you are gathering to be together for the sail away, I would suggest that you all gather on Deck 10 near the bow. It will have the best view as you pull out of port.
Oh and a certificate for $100 credit at the casino would be a great welcome bag stuffer!!!!
Explorer of the Seas October 2013
Caribbean Princess July 2006, May 2010 & November 2012
Monarch of the Seas November 2008
Crown Princess November 2007
Celebrity Zenith November 2005
Enchantment of the Seas August 2004
Depending on how much you want to spend, naybe a t-shirt, button, or cap with something on it that has to do with the group. I've seen family reunions on cruise where they had done this and it looked like fun.