For very full trips (perhaps 11 or 12 total), it can be helpful to bring along a pair of walkie-talkies. Ask for a volunteer or appoint a rear guard, someone that you think is likely to be conscientious about this. Position yourself near enough to the head of the long line of hikers that you can control things like ensuring everyone is together at an intersection, taking breaks, making sure you don’t pass water sources without considering whether folks might need some, dealing with any sort of trail hazards, etc. When the group is large enough, it’s just not possible to know if someone farther back needs to stop for some reason. Walkie-talkies can give you a degree of “peace of mind” while preventing potentially bad group splits.