I'll just chime in with our experience. We took our then 19 year old, and the first night the "group" met with the
DCL CM that was supposed to facilitate the group. There were about 11-12 over 18's there. They did lots of things, and we only saw our son at dinner and most nights he was coming back to the room around 2 am.
They had lunch with ship's officers; participated in the Pinewood Derby type race (they made race cars out of veggies); every night they went to the adult show in the lounge and then "hung around"; midnight swims. Often the group would just meet up and hang out, sometimes in smaller groups of 4-6 (not always the same 4-6 people) and went to the sports deck.