Hur lägga till en user i en grupp?

       -g initial_group
              The  group name or number of the user's new initial login group.  The group name must exist.  A group
              number must refer to an already existing group.  The default group number is 1.

       -G group,[...]
              A list of supplementary groups which the user is also a member of.  Each group is separated from  the
              next  by a comma, with no intervening whitespace.  The groups are subject to the same restrictions as
              the group given with the -g option.  If the user is currently a  member  of  a  group  which  is  not
              listed, the user will be removed from the group

# vi /etc/group