How do I a user to the Apache group called www-data under Ubuntu or Debian Linux server operating systems?

You need to use the useradd command to add a user to the group called www-data.

Add a new user to the www-data group

In this example, add a new user called vivek to the www-data group, enter:

sudo useradd -g www-data vivek
 
### set the password for vivek user ###
sudo passwd vivek

To verify new settings, enter:

id vivek
groups vivek

The group name www-data is the user’s initial login group i.e. vivek has been added to www-data primary group only.

www-data supplementary groups membership

To add a new user called vivek to supplementary groups called www-data and ftp, enter:

sudo groupadd vivek
sudo useradd -g vivek -G www-data,ftp vivek
sudo passwd vivek

To verify new settings, enter:

id vivek
groups vivek

Add a existing user to www-data group

Type the following command to add an existing user called vgite to the www-data group, enter:

sudo usermod -a -G www-data  vgite
id vgite
groups vgite

Above command will add the user vgite to the supplementary group called www-data. Use -a option only with the -G option.