[a] useradd command – Create a new user or update default new user information or add a new user to secondary group.

[b] usermod command – Modifies the system account and make changes to existing user accounts.

First, login as the root user

You must login as the root user. You can switch to the root user by typing ‘su –‘ and entering the root password, when prompted. However, sudo command is recommend under Ubuntu Linux for switching to root user:

su -

OR

sudo -s

OR

sudo useradd ...

Ubuntu Linux: add a new user to secondary group

Use the following syntax:

useradd -G Group-name Username
passwd Username

Create a group called foo and add user tom to a secondary group called foo:
$ sudo groupadd foo
$ sudo useradd -G foo tom
OR
# groupadd foo
# useradd -G foo tom
Verify new settings:

id tom
groups tom

Finally, set the password for tom user, enter:
$ sudo passwd tom
OR
# passwd tom
You can add user tom to multiple groups – foo, bar, and ftp, enter:
# useradd -G foo,bar,ftp tom

Ubuntu Linux: add a new user to primary group

To add a user called tom to a group called www use the following command:

useradd -g www tom
id tom
groups tom

Ubuntu Linux: add a existing user to existing group

To add an existing user jerry 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 jerry
id jerry

To change existing jerry’s primary group to www, enter:

usermod -g www jerry

For more information and options read the following man pages:
useradd(8)
usermod(8)