I've made a small patch which allows to put default groups on /etc/default/useradd instead of specify them every time using the -G flag while calling useradd. e.g: adding EXTRA_GROUPS=restricted,noserver to /etc/default/useradd equivals to -G restricted,noserver
Created attachment 18395 [details, diff] extra groups patch
this is something to go upstream and not for us to add imo
please send this upstream if you still have need of it.
ive already sent it upstream, so the author is aware of it already