Create a new group (www-pub) and add the users to that group

groupadd www-pub
usermod -a -G www-pub usera ## must use -a to append to existing groups
usermod -a -G www-pub userb

groups usera ## display groups for user

Change the ownership of everything under /var/www to root:www-pub

chown -R root:www-pub /var/www ## -R for recursive

Change the permissions of all the folders to 2775

chmod 2775 /var/www ## 2=set group id, 
                    ## 7=rwx for owner (root), 
                    ## 7=rwx for group (www-pub), 
                    ## 5=rx for world (including apache www-data user)

Set group ID (SETGID) bit (2) causes the group (www-pub) to be copied to all new files/folders created in that folder. Other options are SETUID (4) to copy the user id, and STICKY (1) which I think lets only the owner delete files.

There's a -R recursive option, but that won't discriminate between files and folders, so you have to use find, like so:

find /var/www -type d -exec chmod 2775 {} +

Change all the files to 0664

find /var/www -type f -exec chmod 0664 {} +

Change the umask for your users to 0002

The umask controls the default file creation permissions, 0002 means files will have 664 and directories 775. Setting this (by editing the umask line at the bottom of /etc/profile or /etc/login.defs) means files created by one user will be writable by other users in the www-group without needing to chmod them.

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Source: What's the best way of handling permissions for apache2's user www-data in /var/www?