# Create new user - useradd
# -s SHELL    : The name of the user's login shell
# -m          : Create user’s home directory if it does not exist
# -d HOME_DIR : Home directory of the new user
# -c COMMENT  : Some description of user
# -g GROUP    : Group name of the new user
#    USERNAME : Name of the new user
#
# And here is an example:

useradd -s /bin/bash -m -d /home/some-new-user -c "John Snow" -g root some-new-user

# Then set password:
passwd some-new-user

# Delete new user with his $HOME:
userdel -r some-new-user

# Get users list
awk -F: '$6 ~ /\/home/ && $3 >= 500 {print $1}' /etc/passwd

# Create new group
groupadd groupname

# Delete group
groupdel groupname

# Add existing user to existing group
usermod -a -G groupname username

# List of groups user joined
id username