Actually, the best type of group cruise is a TA. They can be as short as 12 days, less expensive per day than Caribbean and airfare oneway. Airfare from say choiceair from just about anywhere in the US to the popular Euro ports will be $450 to $650. Add the airfare to the TA cruise price and it will not be anymore than the Caribbean. As an example, I have a 15 day balcony TA booked for 11/1/13 on the new Reflection for $1199 + $575 air +$450 OBC. That's a group rate, but that's what a group rate should be.
What makes the TA a good choice is the group activities with the CM posters that you have been corresponding with for so long. Groups always have slot pulls, cabin crawls, sailaway and wine tasting parties, grab bags and many more social get-to-gethers. Also there will be a few port stops in historical Euro cities, much different than the carib.
For those who cruise alone, you have been corresponding w/others on this site long enough where sharing a cabin with another single you may not have met but know from sharing discussions will benefit both.
O F C'er