Good Linux question — answering as the UphillTech team:
The utility is usermod and is used like: usermod -a -G group1,group2 username Where username is the user you want to modify and group1 and group2 are the new groups you want that user to join. Running the command without the -a argument will remove that user from all groups except group1 and group2 . To check a users group memberships use the groups command: groups username
Full answer by quackingduck: https://superuser.com/a/95976 (CC BY-SA 4.0).
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