usermod -G <secondary_group> <username>
eg.usermod -G sales saagar
This command has a problem...If you want to add one more secondary group to saagar, say culturals group, then
if we do
usermod -G culturals saagar,
and now if you type
then you will see only culturals as saagar's secondary group and sales is lost.
so i think follwing is the better way:
gpasswd -a saagar sales
gpasswd -a saagar culturals.
now saagar will be in both sales and culturals secondary groups.