This is one basic manipulation I always seem to forget. Here goes.
a user to a group or a series of groups, I normally use usermod with the -a (append) switch like this:
# usermod -a -G group1,group2 user
Let's say I want to add user joe to the lp group, I'd do:
# usermod -a -G lp joe
To add him to the lp and scanner groups, the command would be:
# usermod -a -G lp,scanner joe
Now what's the canonical way to remove
a user from a group or a series of groups? This is the sort of command I rarely use and always seem to forget.