Add users and join groups

usermod -a -G groupA user #Add user to groupA group (must add -a, otherwise user is just groupA group)

usermod -a -G sudo jack so that jack has sudo permission

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See where a user belongs Several groups
#groups jack
jack : jack sudo


#sudo useradd -s /bin/bash -G sudo -d /home -p `openssl passwd -1 -salt "12345678" testing` sarcky # Add username sarcky and password testing user.