The more riders on the horse, the more likely it will be to have confusion. Yes. If all riders have a different approach. HOrses can deal with this just fine. But, if you are trying to erase a bad habit, or sharpen certain behaviors, you must have all participants on board. For example, be sure that all agree not to allow grazing under saddle, or to reinforce the "halt" command with a backup if necessary, or to reprimand any bad ground manners , or . . ?
And, you may need each person to commit to certain days, and not change without mutual permission.
When leasing, the leassor pays a fixed amount, whether they actually ride or not. If they dont' ride, it's their own choice. Sometimes I pay for my leaase and only get a few rides in a month (due to bad weather), other times, I ride a lot. Them's the breaks.