I'm not sure this is easily knowable. I know that many times that group I am with, while simultaneously being slowed down by my uphill pace, also typically limits that amount we bite off, but sometimes the group is up for anything and no one wants to say no. I can also imagine scenarios where the group has a wide range of abilities, so some members of the group are being pushed, while others are being restrained. I would think that the chances of injury increase with the difficulty of the trip, independent of group size. Weather the group influences the agenda is a much messier item to discern. Maybe some SAR vets can offer a good anecdote.
Yes, what TJ said. (the very short version)
Now for the long:
Define group? You could pull three or four people from here (probably 15 or 20 groups of 3 or 4) that would consider a winter Presi-traverse to be a fun day. Different people push (or don't) push themselves and some push or don't push weather. Defining bad weather may be harder than defining a group. (Are meet-up's a group?) If you start as a group of 30 - remembering some novice groups have not learned about group size (or choose to ignore it - possibly some meet ups) should you expect all 30 to stay together or will they create a fast, moderate & slow group based on their speed, out of neccesity? You may or may not have the experience of the group evenly divided that way.
One of the things I like to know when I've interviewed either leaders or people to hike with is to learn about a couple of trips where they failed to reach the summit or goal and why. I'm convinced you learn more on trips you'd consider a failure (talking local generally, not an 6, 7 or 8K peak. Starting at 10:00 AM for a presi-traverse and bailing at Washington is different than doing the same thing due to weather.
You should be comfortable with the pace, the weather, gear decisions and destination. Your first trip with the leader shouldn't be a winter traverse, Presi, Bonds, or Pemi. (If Franconia, need to be comfortable with the weather.) Once upon a time winter AMC trips required Sorel type boots or plastics. Probably are still a few hold overs. Knowing when the leader will turn a group around is important too. (Some have a summit or die mentality, some bale soon after the clouds start to form and there is a wide range between these too extremes)
Is there a leader? Several years ago, I organized some hikes with other people from VFTT. I considered myself the planner, not the leader & I made that clear. Most of the people were pretty strong & in most cases knowledgable too. (or vicea versa) Don't remember having anyone turn back, I was one of the slowest of the group, sometimes someone would stay at a hut & skip a summit spur. I think everyone was strong enough that if they were unhappy with the group decision, they were comfortable heading back on their own.
(We often started as a group other than Chomp as he liked sleeping in some and was the fastest of the group, especially around the time of his AT trip. He was able to make up roughly an hour & catch us before we got to treeline. I did get to the Tecumseh summit before he caught me once when we did a loop including the two Osceolas, on the others, he caught me before the top.)
Seasons do make a difference. It's hard to make enough bad decisions in the summer to put your life in dire jeopardy. (Okay descending Huntington or 6H or other headwall trails in the dark without lights & with novices or T-Storms with metal poles & standing on Laf, Lincoln or another high peak...) In the winter, a late start & leaving the face mask & goggles in your ski bag on a very cold day may be enough to start the process of losing a piece of your nose or having vision issues that make you have to stop.
