I've never taken a group cruise, though I would not be adverse to one.
On one HAL cruise about 300 people had booked through one TA, and one evening they closed off the entire Lido pool area from 5 to 8 p.m. so that they could have a cocktail party. That was annoying.
On my transatlanic last year, a passenger had asked that a needleworker's get-together be posted in the daily program, and about 30 people showed up; the cruise director noticed, and every following morning until we reached Lisbon there was a coffee service and pastries or cookies of some type waiting for us. Nice!
On this last one there were a group of about 14 who signed up with Craft Cruises, for the cruise and knitting lessons each day. I was not with them but spent some time with them in the area of the ship that had the best lighting; and may very well take a future cruise with one of them.