Create a group in Linux with command line :
sudo addgroup nginxusers

sudo usermod -a -G nginxusers newusername

The information with the list of groups is stored in the file /etc/groups
You can view it with: nano /etc/groups (with sudo)

sudo adduser newusername nginxusers

To list usernames on Linux using the awk interpreter, run the following command
$ cat /etc/passwd | awk -F: '{print $1}'

sudo chgrp nginxusers -R www/