shell
usermod

aggiungi un utente a un gruppo

diciamo che hai due utenti con nome utente chyingp e casper e vuoi aggiungerli entrambi al gruppo root .

usermod chyingp -a -G root
usermod casper -a -G root

ecco che arriva la spiegazione di -a , -G

-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.