The collars probably have spikes on the inside to discourage the dogs from pulling.
At least, that is what I would do.

Sure there could be, and no doubt is somewhere, developed training for it. Certainly there is a skill, both for dog handling, and for leading groups of fifteen.

The dogs also have some training in behaving appropriately in general with other dogs and other leash-holders – I’m sure some dogs would have more problems than others.

Moreover, I think it helps immensely that dogs naturally tend to relate to being in a pack.

And of course, there are dog sled teams…

Most professional dog walkers won’t take on four-footed clients that are poorly behaved.

The key word is likely “park.” If the dogs are being walked within a confined area without traffic, there are very few dangers. The pack might get a bit unwieldy and spread out, but there’s no risk of dogs leaving a sidewalk and get hit by cars.

But, I have to wonder how the dogs get to the park. Does their walker need to take them through risker terrain to reach and leave the destination? I would be concerned about that.

Single, married – not really a relevant criterion.

They use a leash coupler that allows them to use one leash & allows the dogs to feel like a member of a pack. The dogs work together while the walker is the Alpha of the pack.

Enlightening answers of course. Thanks jellies!