I've only been "back in the saddle" for 4 years now so not as much experience as other posters but I'll throw out my opinions anyway.
My first large group/charity ride (around 40 bikes) was a disaster. No police escort was arranged for so the pre-ride brief included the expected "we will obey all traffic laws, etc." Well, we pull out of the Wal-Mart parking lot, first traffic light turns yellow, me and one other guy stop, and about 20 more bikes roar through the light. We catch up as the group pulls onto the interstate for a 10 mile or so stretch. We're doing 80 or so and BARELY catching the group. Get off the interstate onto a nice two lane road and, again, having to do at least 15 over the posted speed limit to keep up. Then I get stuck behind a guy with the loudest pipes you've ever heard. End of the story is when, and this was through none of the riders' faults, there was a terrible accident and 2 guys were killed.
Sorry to ramble on about that but it's why I'm obviously not a big fan of large group rides, especially with no LEO escort. WAY too much depends on other people's skills, attitude, and leadership abilities to be able to relax and enjoy the ride.
I'm sure everyone here will admit that part of the attraction to owning a bike is to be able to get on, fire it up, and get away from everyone and everything else for awhile. I've had some absolutely great and memorable solo rides. Doesn't matter if it's 4 hours or 4 days.
Yet, it's still a blast to get together with a couple other people who ride the same as you and share the comraderie of the road! Other than solo, I've ridden with up to 2 other people. I guess a few more than that would be fine but no huge groups for me. Unless Biketoberfest counts as a 100,000 bike group ride!