usermod command
the default path of the command is /usr/sbin
add one user to one group:
here use the option '-a'
from the manul
-a, --append append the user to the supplemental GROUPS
mentioned by the -G option without removing
him/her from other groups
/usr/sbin/usermod -a -G group1 user_name
remove one user from one group, you have to move that user to another group:
/usr/sbin/usermod -G group2 user_name
