There is a section on the DCL web site about family reunions but that is just about special t-shirts, pictures, etc. Actually has an additional cost associated. Didn't see anything about group discounts.
We did a family reunion DCL cruise last May and it turned out great! We had 10 in our party including grandparents/sisters/brothers/nephews and nieces. We had four staterooms (2 Cat 10s and 2 Cat 9s) on deck two. This is a terrific way to do a large group vacation. I highly recommend it even if you can't get a discounted rate.