The group site set apart from the rest of the camping, so you can really enjoy the space without feeling like you are bothering the other campers. There's lots of space for tents plus some wooden bunks in a wooden shelter, a nice option for people new to camping who don't have a tent, but I'd be afraid of spiders! :O Site is adjacent to a river access, which I did not get a chance to check out myself, but being near water is always a plus to me.
The campgrounds are about an our of of Seattle, so it's a really nice low-commitment option for those residing in the greater Seattle area. Despite being so close, it doesnt feel very populated or anything like that, nice and woodsy still! :)
The only drawback to the group site is occasionally you'll get a random person wandering through to access the water.
I wouldn't suggest going so late in the year unless you are prepared for rain, it poured on us most of the stay, but we still managed to have fun. Luckily there is a large covered dining area so it's survivable in rain