The following command will have to be executed by a root user. Replace usera with the username that you want to grant root privileges to.
Warning: commands that are executed by the root user can do a lot of damage to your system if you do not know what you are doing. Please pay attention and be careful with what commands you execute as a root user.
pw user mod usera -G wheel
After you have successfully executed the command. The user that you just added to the wheel group should be able to become a root user by executing the command su inside a terminal.