If you are getting the following error message when you create a boundary group in the Technical Preview 1609 release then fear not. This is a known issue and there’s an easy fix for it.
Just go to the General tab and add in a boundary that you want assign to the group and click OK.
Then when you go back to the References tab all is good.
Now can start to check out the new style boundary groups. MS may release a hotfix for this problem, in the meantime there’s the workaround.
My fellow WMUG buddy Robert Marshall has a deep dive into the new Boundary Group functionality here and it is well worth a read.