To see lists all groups that a user is in, run
groups USER_HERE
Example
boby@sok-01:~$ groups boby boby : boby adm cdrom sudo dip plugdev lpadmin lxd sambashare libvirt docker boby@sok-01:~$
To add a user to a group, run
adduser USER_NAME GROUP_NAME
Example
root@vagrant:~# adduser boby sudo Adding user `boby' to group `sudo' ... Adding user boby to group sudo Done. root@vagrant:~#
To remove a user from a group
deluser USER_HERE GROUP_HERE
Example
root@vagrant:~# deluser boby sudo Removing user `boby' from group `sudo' ... Done. root@vagrant:~#