We had this as a constant issue when I worked in a school with a strong outdoor program.
Slow people in front. In extreme cases, start them off 10 minutes early.
Slow person with a pacer. Individual behind the slow person encourages and gives tips.
Designated sweep. This is a responsible (and strong) individual who is last, and makes sure that people don't evaporate off the back end of the line.
Group leader generally travels at just sight range of front of group. So with a 25 person group, he may travel about position 8-10. When I ran trips about once an hour I would work my up to the front of the line, then drift back chatting for a bit with each person and looking at them to see how miserable or happy they were. Since it's a lot easier to go back than go forward my favorite position was to be where I could just see the first person.
System of whistle signals.
Two long blasts from behind. Stop the front of the line. Acknowlege with two short.
Two long blasts from front. Where are you? Acknowledge with two short. Used when you weren't sure how far back the other end was.
One long blast from behind. Proceed. Ack with two short.
Four long blast from behind. Proceed, but slower. Ack with 4 short.
We used fox40 whistles which are very loud, but unfortunately sound a lot like marmots. Hence all the 2 toots.
Two others we had in our list, but didn't use normally:
Three long blasts: "Come to me!" Three shorts "I'm coming" Three blasts is a distress signal. We tried not to use it unless necessary.
On lakes we had designated lead, point and sweep. No one went ahead of the lead, further from shore than point, or fell behind sweep.
Rivers were harder. Worst case was a 12 canoe brigade. The problem with rivers and small stream work, is that if a canoe gets wedged on a rock, you can get a pile up. We never had this actually happen, but we talked about it a lot.
On rivers we traveled in groups of 4 canoes. A group generally traveled in yelling distance, although everyone had a whistle to get attention. Usually the 2nd most experienced or skilled person would lead. He most likely to not get in trouble, and best able to say if this was beyond the group's skill. It also meant that there was someone at the bottom of the rapid to fish out the debris if someone dumped.
The last canoe had a highly skilled person too. He was the one who would have to dodge wedged canoes.
Groups traveled about 5 minutes apart. When we got to the next lake or long reach we would pause and regroup.
Group was split in half with one half going ahead on snowshoes breaking trail. The mushers broke camp, and would set out an hour later. We treated the two groups as being independent, sometimes not seeing each other all day. More commonly the dogs would catch up in a bit over an hour. Mushers would halt, and then catch up.
If the terrain was rough, mushers would have to park a sled and help the guy in front or behind get up the hill/over the creek bank, around the deadfall. On occasion, breakers would have to come back and help the sleds. On rare occasions the breakers got too far ahead.
Keeping the sleds in a group wasn't a problem. Dogs will go faster if there is a team in front of them.