samba - Aug 2018 - using Windows AD unwanted Group rights get applied to new Files

> By default, every AD user is a member of 'Domain Users' and so, 
> when you use the 'rid' backend every Unix user gets the group as
> their primary group.
> The only way to change this is by using a version of Samba >= 4.6.0 and 
> use the 'ad' backend  (...)
You can also use RSAT and define some other group as the user's primary
group, and still use 'rid' backend. If I remember well, the setting
resides in the "Member of" tab of Active Directory Users and Computers
(ADUC).