User and group creation

_$: adduser  <user>
_$: addgroup <group>
_$: usermod  <user> -aG <group>  # Add user to group
_$: gpasswd -a <user> <group>    # Add user to group
_$: gpasswd -d <user> <group>    # Remove user from group

User and group rename

_$: usermod -m -d /home/<new-login-name> -l <new-login-name> <old-login-name>
_$: usermod --move-home --home <new-login-name> --login <new-login-name> <old-login-name>
_$: groupmod -n <new-group-name> <old-group-name>
_$: groupmod --new-name <new-group-name> <old-group-name>

Add a user that has the same name as an already existing group

_$: useradd <user> -g <group>   # Use useradd rather than adduser
_$: passwd  <user>