add user as group member

[jeffry s]

i know this question sound so stupid.
after gooooooogling. i end up with useradd, groupadd, usermod, userdel,
groupmod, groupdel, groups.
i do not be able to determine how to add a user to particular group in the
system.
i use to do a manual edit on the /etc/group files and it work fine so far

it is just a matter of curiosity, is there any command out there i can use
for this task?

thank you..