It ended up being 4 planes - the main organizer in the Cessna 172 he owns a share of, the pilot I was with in a rented Diamond DA-40, a former flight instructor in the Piper Cherokee he owns, and a couple who are both pilots in their Glasair. In addition to the 5 pilots there were 5 passengers - me & my wife were in the Diamond, two others in the Cessna and 1 in the Piper.
The flight up was fun but uneventful, the pilot let me have the stick for about 10 minutes with the basic goal of just flying us along straight & level and avoiding the other planes in the air. The Diamond's electronics are so far ahead of what's in the other planes that it's kind of scary. It showed on the map view every other plane within about 10 miles, and it was disturbing to me that they had to get pretty close before I could spot them. Without the display telling me that they were 3 miles away at my 3 o'clock, 300 feet above me, how well would I really spot them and know that there was a plane getting close? This is one of the things you learn when getting pilot training, and I definitely got better at it even from beginning to end of the flight, which is turned out to be a good thing... We'll get to that.
Got to York, and landed pretty much right away in spite of the airport being rather busy that morning. Met up with everyone and had a pleasant brunch. We went out and admired the Glasair, the fastest of the planes that flew up and the only two-seater, then loaded back into our respective planes for the flight home.
On the way back the Glasair left for Frederick, and the other three planes ended up flying in loose formation. Everyone throttled back to match speeds with the Piper and took up positions relatively close to each other - perhaps a half mile separation, plently close enough to see each other's planes. The passenger in the Piper got some great photos of the other two planes.
This was made hugely easier for everyone by the Diamond electronics - we were able to see clearly that we were level with and a little ahead of the Cessna and drop back slightly to come alongside, and could match speeds with the Piper more easily too. It ended up in a triangle formation with the Piper in the front, and about 200 feet lower.
All's going well, we're tooling along through relatively calm clear air just below the clouds, I'm glancing back and forth between the view and the electronics to see when other planes come into view... when one of my glances forward causes me to say, "Um, glider, dead ahead!"
A sailplane pilot had seen us in formation and was coming in to buzz directly between us in the opposite direction. I don't know if he hadn't seen the Piper or just knew he was high enough above it, but he went almost perfectly halfway between the Cessna and us. Total time between spotting him and being past each other was maybe 30 seconds.
The Diamond's system shows other planes on the display based on their transponder signals. The sailplane didn't have a transponder. The first warning we had of it was when I noticed it. I'd gotten too comfortable in feeling like I knew everything that was up in the sky with me. It's possible the other pilots, not relying on instruments, saw him earlier - but not by much. We guessed he'd decided to hotdog and show off for the boring old normal planes, and may never have even seen the third one that was a little lower.
Coming in for a landing we got a bit more of a feel for what it's like to be in a crowded airspace. A fourth plane came in from the west as we were circling to land, and the Cessna (in the back by then) had to circle again to give him some room to land. On the plus side, I was the first to spot the other plane (having been told by the electronics where it was) and also spotted a small plane far below that was towing another glider aloft and didn't show up on the electronics. I was getting better at spotting planes against the background, whew!
Then, right as the Cessna was about to finally land, someone who either was oblivious or having radio problems missed all of the radio conversation about which direction people were landing from and announced that he was coming in to land the opposite direction, then ignored a correction message about which direction he should be landing from - so the Cessna revved back up to full throttle and aborted landing, went around again and landed after that guy was off the runway.
As one of the pilots said afterward about both the glider and the wrong-way landing, "Every so often we get a reminder that the same people flying up there with us also drive on the beltway."