Usermod/useradd

Add existing user tony to ftp supplementary/secondary group with usermod command using -a option ~ i.e.

add the user to the supplemental group(s). Use only with -G option :

# usermod -a -G ftp tony

Change existing user tony primary group to www:

 # usermod -g www tony

Use useradd command to add new users to existing group (or create a new group and then add user). If group does not exist, create it. Syntax:

 useradd -G {group-name} username

Create a new user called vivek and add it to group called developers. First login as a root user (make sure group developers exists), enter:

 # grep developers /etc/group

Output:

developers:x:1124:

If you do not see any output then you need to add group developers using groupadd command:

# groupadd developers

Next, add a user called vivek to group developers:

# useradd -G developers vivek

Setup password for user vivek:

# passwd vivek

Ensure that user added properly to group developers:

 # id vivekOutput: 
uid=1122(vivek) gid=1125(vivek) groups=1125(vivek),1124(developers)

Please note that capital G (-G) option add user to a list of supplementary groups. Each group is separated from the next by a comma, with no intervening whitespace. For example, add user jerry to groups admins, ftp, www, and developers, enter:

# useradd -G admins,ftp,www,developers jerry