Pros: Why travel with one BFF when you can get the whole gang in on the action? A group vacation can be double or triple or quadruple the fun, and is a great way to strengthen old friendships. Plus, with more people, if two of you want to check out the art museum while three of you would rather hit the beach, you can split off and reunite for dinner.
Cons: More people means more personalities which means more potential for arguments and complications. And that whole "splitting off into small groups" thing is great, unless you're the odd man out.
Take the trip if... social time is your first priority and you don't mind compromising on day-to-day plans.