1. Linux

Use id’s in the 1000-1099 range for normal user accounts.
Use id’s in the 1100-1199 range for special user accounts.

1.1. Add user account

Enter the following commands at a Command Line.

# Show all user id's that already have been used.
cat /etc/group  | grep -E '1[0-9]{3}'
cat /etc/passwd | grep -E '1[0-9]{3}'
# Show highest normal user id used.
cat /etc/group /etc/passwd | awk -F: '($3 >= 1000 && $3 < 1099 && $3 > max) {max=$3};END {print max}'
# Show highest special user id used.
cat /etc/group /etc/passwd | awk -F: '($3 >= 1100 && $3 < 1199 && $3 > max) {max=$3};END {print max}'

Enter the following commands at a Command Line.

sudo useradd --comment <User> --create-home --shell /bin/bash --uid <uid> <user>
sudo passwd <user>
sudo adduser <user> sudo                # Allow usage of sudo
sudo gpasswd -a <user> vboxsf           # Only for virtual machines

1.2. Remove user account

sudo userdel --remove <user>
sudo rm -rf /home/<user>        # Not needed ???