Ubuntu Linux add a user to group command

The procedure is as follows:

1. Open the terminal application.
2. Login to Ubuntu server using ssh.
3. Add a new user named foo to www-data group by running useradd -g www-data foo command.
4. Add an existing user bar to Apache’s www-data group on Ubuntu using usermod -a -G www-data bar command.
5. Verify new modification on Linux by using the id command.

Let us see all commands in details.

How to see a list of www-data members

To see list of all members of a group named www-data, run the following grep command on /etc/group file:
grep ^www-data /etc/group
Sample outputs:

www-data:x:33:

How do I add an existing user named vivek to group Apache group www-data?

Simply run the following command to add an existing user to an existing group:
sudo adduser {USER-NAME-HERE} {GROUP-NAME-HERE}
sudo adduser vivek www-data
Verify it with the help of id command:
id vivek

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.

Conclusion

You learned how to add a user to group www-data on Ubuntu Linux using command-line option. For more information see useradd, and usermod man pages online here or by typing the following man command:
man usermod
man useradd