Lets say we need to add a new user and grand him root privileges.
Use the following commands to create the new user sam, grand him the same privileges as root and set him a password :
useradd -ou 0 -g 0 sam
passwd sam
We've just created the user sam, with UID 0 and GID 0, so he is in the same group and has the same permissions as root.
To check the entry
cat /etc/passwd