usermod(8) -aG plugdev $LOGNAME
modify a user account
-a, --append
    Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
    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. This behaviour can be changed via the -a option, which appends the user to the current
    supplementary group list.
usermod [options] LOGIN

The usermod command modifies the system account files to reflect the changes that are specified on the
command line.
source manpages: usermod