AFAIK, there can be only one root user. Adding an ordinary user and adding them to the sudo group is one way of allowing a user to access root privileges. I don't know what the effect of adding them to the root group would be.
If you're worried about the root account being compromised, you could try resetting the password for the root account. Then create a user with sudo privileges and use that account to disable the root account and then perhaps limit the permissions of the rogue account you spotted, or simply remove it altogether, if you haven't already!
thanks, but the point is if i add one user abc, password 1234 and use
usermod -a -G sudo abc, but user abc don't know root password, he can't have root priviliedges, my goal is to let user abc to have all root priviledges without knowing root password