Managing users and groups in Linux

Usergroups:

https://www.tutorialspoint.com/how-to-add-a-user-to-the-group-on-linux

Listing users and groups:
https://devconnected.com/how-to-list-users-and-groups-on-linux/

When no www-data user:
https://unix.stackexchange.com/questions/58157/what-is-the-equivalent-for-www-data-for-centos

For non www folders:
https://www.techtrekking.com/how-to-resolve-error-authentication-is-needed-to-run-bin-cp-as-the-super-user-error/