This is just a heads up.
A recent change in mythtv/packaging relating to gksu has broken mythtv-setup and mythfrontend mythtv group membership settings on first run for clean installs from ppa:mythbuntu/0.29 and 30.
Originally I thought it only affect Xubuntu 18.04 which is still in beta, but the effect is much wider xubuntu 16.04 kubuntu 18.04, Debian Stretch and possibly others. I suspect the code has been broken for some time, with the change relating to gksu exposing the issue.
A quick fix for xubuntu and debian stretch is to install gksu (sudo apt install gksu)
For details please see mythtv trac https://code.mythtv.org/trac/ticket/13256
Edit:
As of Xubuntu 18.04 beta 20180421 gksu is no longer available so mythtv user cannot be setup.
Mike