Q. How do I add a new user called foo under existing group bar?
A. You need to use useradd command, which create a new user or update default new user information. You need to pass -g option to useradd command. It specifies the group name or number of the userÃ¢â‚¬â„¢s initial login group. The group name must exist. A group number must refer to an already existing group. The default group number is 1 or whatever is specified in /etc/default/useradd file.
Task: Make sure group sales exists
# grep 'sales' /etc/group
Task: Add a new user tom to group sales
# useradd -g sales tom
# passwd tom
You can add existing user jerry to group sales with usermod command:
# usermod -g sales jerry
You can add existing user jerry to secondary group purchase with usermod command:
# usermod -G purchase jerry
Please note that above commands works with Solaris UNIX, FreeBSD and UNIX like oses.