We track live availability at 42 campgrounds with group campsites in Illinois, across 40 towns. Group sites are booked further ahead than anything else — reunions, scout troops and weddings plan a year out — and there is rarely more than one or two per campground. A cancellation on a group site frees up a whole weekend for a whole party, which is why they do not sit unclaimed for long.
Booking goes through Illinois State Parks and Recreation.gov. Searching is free.