We haven't done it with a large group of adults. Two years ago we chartered a moorings 4000 with another couple and it was a blast. Yes definitely cheaper, halving costs basically) We know this couple quite well and have travelled with them and their kids before and other than difference of opinions in regards to air con/ non air con there were no issues (not that this was an issue really!)

I would say it is best to know whether you and the rest of the group would get a long because it is tight quarters. Obviously the larger the group the more difficult to find compatibility.

Keep in mind too that the larger the group the larger the boat required. So if you are going from a 35-40' ($3500) monohull for two people to a 48' Cat ($10-12k) for 8 people how much are you saving really especially after the hassle of arranging everything? My 2cents anyway.