If they have their own shop signs then you still can't multiple people adding stuff to them. In addition to that the pwarp maintenance is still only being pulled from one person so the other people would need to send payments to that person rather than it all just funneling to the pwarp/shop owner.
If they don't want others to be able to open the chests it's easy enough to turn off plot permissions for anyone but the plot owner, but if you have a small group of people who want to all work together on a shop that's not currently possible. You end up with extra chests where others put stuff in, so the shop owner can transfer them to the shop chests rather than the other people just putting it directly into the chest.
Then if something is low the shop owner has to communicate that to the gathers rather than them just being able to look and see what's needed.