My 2 cents.
Every single person in a group is responsible for looking back at the person behind them over and over and over. It is not the first persons responsibilty to make sure the last person is there, but only that the second to last person knows that the last person is there. (Unless of course your entire group is 2 people).
Likewise group communication is passed this way. I've seen pins result from information not being passed back. The first person signaled to the second, the second to the third, and then somewhere back the message stopped, and the fifth person ended up being stuffed under a tree.
I often hear that large groups can't navigate a river, and people try to limit group size to four. This isn't neccessary. Last season we had a group of eight that succesfully ran the Embudo at 3.7, and none of the paddlers had eddy problems from having too many people. We simply divided up, sometimes into two groups of 4 (working closely together to run the rapids), and other times into four groups of 2. TG of the Range Life said that they had 9 on every river, and were in gorged out first D runs for a week straight. It was never a problem.
The fact that many groups cannot safely navigate a river with a large group is a sign that most paddlers know very little about how a team should function. More often than not, everyone meets and bombs down together, and then wonders why they are always clustered.
At the very least, when a paddler feels that the person in front of them doesn't look back enough, they need to say something to that person then, not to their own friends later. (Thankyou to all of the paddlers that told me that when I was younger)
I also personally signal for people behind me to move closer when I know what is coming up, and know that it will be easy for them to follow me, and for me to keep my speed. They rarely respond though.
When I'm on steep creeks with small eddies, I prefer the leap frog method...another story.