add user as group member

["jeffry s" <paragasu@gmail.com>]

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..