Posting a little late. I’ve always ponied my horses, from the get go when I’ve started them; I find it very helpful for the young ones, and have liked to pony one while riding on a long trail ride, then switch horses at a break, back and forth. It can be a great way to leg a horse up, and get a greenie out and about exposed to things. I’ve also done such things as ponied a good friend’s horse along on a multi-day ride, when requested, when she couldn’t join us for the first day or so.
Agree with the general guidance given above, although there are a few things I might have done slightly differently when ponying multiple horses at a time, or when riding with a hand horse (an extra horse in hand) rather than strictly ponying. I’ve given demo exhibitions at events, doing various maneuvers with multiple horses, and was fortunate to be instructed by a couple of mentors. One thing that was emphasized to me was how important it was that the ridden horse be respected by the ponied horse(s), and also, when ponying multiples the horses needed to be carefully organized as far as which one was next to which other one (also taking into account relative speeds).
One thing I haven’t seen mentioned yet (unless I missed it), which I personally do, is to have cage stirrups on the saddle, especially with a horse new to being ponied. I use the type with metal toe cages. As important as it is to keep the ponied horse up alongside (rather than trailing the ridden horse), what can happen is that it can end up very (too) close. I like the cage stirrups because they not only protect my toes in such a situation, but I can easily push the ponied horse over a bit with the stirrup. Just a suggestion.
IME, ponying becomes second nature with practice, and I think the horses like it.