A big huge grass field designed for large group camping. There is a large covered pavilion for meals, pit toilets nearby, though you aren’t far from the showers and regular bathrooms of the rest of the state park. There are also a handful of small wooden bivouac-like covered shelters that are available for sleeping.
While there are several camp and RV sites in Beacon Rock State Park, this is the only group site that I know of and it fits up to 200 people at around $4-$5 per person (the website says rates vary by person, but this is the rough price I've heard) with a minimum of 30 people to reserve the site. There's no showers on-site, you can head to the main campground across the road if needed.
Features include two super-cool open fronted cabins (called adirondack shelters), a covered picnic area made from huge timbers, one kitchen shelter with limited power, and scattered picnic tables. There's an enormous meadow that would fit a hundred tents plus areas for RVs as well (though no hookups). Two vault toilets are at the top of the meadow, with flush toilets also accessible a mile or so away at the moorage campground.
Perfect for large groups such as family reunions, but also fun with just 6-8 families since the minimum numbers are pretty low.
Tip: When reserving online, this site is called G1 and requites a minimum of 30 occupants. The cost for 30 people for one night in July was $138.77, or about $4.62 per person. The weekends book up months in advance, but week days are often free.