A great question has come from @sitereport on Twitter:
Can multiple iPhone users add items to the same order?
Technically, as long as the users are logged in to the same account, then yes you can add to the same order (or change quantities or remove the item).
Indeed, this can be a mix of anyone on the iPhone app, the Nokia app, and the web site - each keeps the others up to date.
I say 'technically' because if someone checks out an order, the basket is emptied ready for the next order and other users on the same account are not alerted to this unless they tap the 'basket' button which causes the app to ask our servers what the current basket status is.
Always check the basket contents regularly before adding items if you know someone else is likely to be doing the same thing.