When I went to here (
http://web.myshopperapp.com/app/) and logged in I was able to share a list - at least the friend received the email, but when the link in the email is clicked a message is returned from Safari (on the iPhone) indicating the page cannot be opened, because the address is invalid.
I also notice that not all of my lists are showing on the web version. (Resolved)
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Opening the the email link in Mail on my iMac resolved the invalid link error received when trying to open it on the iPhone. Sharing/syncing seems to be working at this point.
One issue is that there is no feedback one way or the other following the attempt to share a list. It's only after the individual with whom you intended to share the list activates it from their end and the list icon changes that you know anything at all happened.