Per my rating system, I usually reserve 5 stars for those parks that are worth traveling to from long distances (i.e. out of state). While Marin county CA is definitely worth traveling to, it's the region that's 5 stars, not necessarily individual campsites in the many state parks worth visiting in the area.
But this group site is the exception. I've never tried to book this in the peak season, and I'm 99% sure it requires planning years in advance. If you can get this site for a family reunion or just a weekend reliving your college days, you should. And you should encourage your friends and family to travel from out of state for the festivities.
The site has a fully covered dining area, fully plumbed sinks and bathrooms, and a plethora of hiking options in the Muir Woods and Mt. Tamalpais. For those familiar with the area, getting into the Muir Woods can be huge hassle. You have either reserve a parking spot in advance, or take a shuttle into the park. The days of hopping in the car on a random morning and hiking in the Muir woods are long gone. Sure, if you are familiar with Marin, there are all kinds of ways to hike into the park from other trail heads for which parking is not an issue. Alice Eastwood group camp is one such spot.
Those who read my reviews know that I'm a huge fan of camping in the winter in CA, and this was one of the trips that brought me to that mindset. I camped here in late Dec 2018. It was a bit wet - the remnants of a larger storm were still dropping periodical rain on the region. And another storm was right on its heels. But there was a window of 2-3 days before the other storm moved in, so I went camping here. One of the finest nights of sleeping in the woods I've ever had was the first night, when I pitched my small 2-man tent amongst the redwoods and drifted to sleep with the light rain drops hitting the fly of the tent. Sure, it was a bit damp in the morning, but it wasn't cold and the hiking through the redwoods was magnificent. The streams were running, and the undergrowth - particularly the ferns - were a fluorescent green.
Highly recommended. This is a special place.