Often times we need to add a user to a new group to give them permissions to certain directories. This is easily accomplished with the following command:
|# usermod -G GROUP USERNAME|
Well this is a double edged sword as this command will strip any group the user is already in. A little less well known is the ‘-a’ switch that will allow you to append the user to the new group while leaving any previous memberships intact.
|# usermod -aG GROUP USERNAME|
Of course, most of the people I discussed this with, just modify the /etc/group file and maybe a safer practice for you.