you could have a look at "man sudoers" or just set an entry to a file in "/etc/sudoers.d". To give a user with username "steffen" full root access to the machine, you'd have to put
steffen ALL=(ALL) ALL
into a file in the above-mentioned directory.
yes further to Steffen's answer.
There's no real concept of a "sudo user." You just assign sudo privileges to a regular Unix account using the sudoers file(s)
You can see what sudo privileges an account has, by running the following from the account: