Interestingly, I just got back from my wife's family reunion and holidays and there were between 120 and 150 of us. Some were in a motel though, and it wasn't really a camping trip as much as it was a huge party that involved camping-as-accommodation.
I think the largest group that was back country specific was a wheeling trip on the Whipsaw trail near Princeton BC. I think there were around 20 people in 11 Jeeps and we were 150 km (around 90-95 miles) off highway. That was an epic adventure.
None of us had met before the trip, and myself and another guy organized it through our local Jeep club forum. People who committed to the excursion came from hundreds of miles in different directions, met up in 2 or 3 spots, and completed the trip with minimal fuss.
One fuss was that my old Jeep gave up the ghost before we even got to the trail head, so my son and I limped home and exchanged for his lifted Durango. We actually ran the first part of it ourselves in the black of night as the main group had gone ahead, not expecting us to return. We ended up stuck on a rock shelf at 6200 feet and slept right on the trail. Morning's light saw us unstuck quickly and back on our way. We were never so happy to run into a bunch of Jeeps miles from nowhere!
A small group of us stayed in the bush an extra night and day, and have become good friends. We still camp and explore together.