I totally agree Hils.
As I have said before, on my first holiday to Cilento Coast there were four people , who although they had booked independently, knew each other from a previous holiday, and although they were fairly friendly, still tended to do everything together as a group to the exclusion of everyone else. We were asked to rotate tables on that holiday, which is a great idea, although most of the holidays that I have been on now, tend to have big tables so everyone sits together. These four never rotated tables which was a shame.
The only other time I have hit the problem was on a holiday in Tuscany, where we had several small cliques, but one group of five ladies, who booked together through a travel agency, claiming to not know that it was a singles holiday company, and were just taking advantage of the no single room supplement. They did not mix at all, and it was very awkward if you were sitting next to them at evening meal, because they totally blanked you.
It's such a shame, and I know that there is not a lot JY can do about it, people are generally very friendly and mix, even if they come with a friend or relative, but there is always the odd exceptions.