gistfile1.txt

       -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.