I am trying to transfer an app of mine into my company iTunesConnect account but are getting the error:
You can't transfer this app because of the following reasons:
Sandboxed Group Container
You can only transfer sandboxed apps that are not sharing a group container.
the thing is, my app isn't using the group container any more, it was in an older version but not the currently shipping version in the Mac App Store.
does anyone have any experience with how to go about transferring the app?
It appears to be impossible. I got the following response from iTunes Connect Support (September 15, 2017):
By completing this process (a "binary transfer"), the app will effectively be a new and separate app on the Mac App Store. This might of course be very inconvenient, but there's not really anything to do about it, unless Apple changes the rules.