So until now, I seem to have done everything correctly. Now, when I start Dolphin, navigate to /srv/files/documents, right-click and select New Text Document (or any other of the available file types), it create a new empty file with group set to team and permission set to 644 - which is not what I want.
O Dolphin copia os arquivos de modelo pré-instalados , incluindo as permissões de arquivo (0644). Para modificar as permissões no Kubuntu 17.10, execute:
sudo chmod 0664 /usr/share/templates/.source/*
Novos arquivos serão criados com as 0664 permissões.