Updating this topic for the latest features: you can now see when (date / time) users entered and last modified their items by mouseover of their name on the Group order page. (also Splits page, just for split items)
This has been available for a few months, so most users probably would have figured this out by now, but perhaps not the iphone / ipad users who don't have mouseover ability. You have to be on a real computer to see mouseover text I think.