Q. How do I add a new user called foo under existing group bar?

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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 user’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.