Create user
Login as root
$ sudo su -
$ useradd -m <userName> -p <password>

Add to the group
$ usermod -aG <someGroup> <userName>

For granting sudo or root access
visudo
For the group created
<someGroup>        ALL=(ALL)       ALL
$ su <userName> -
$ groups
  <userName> <someGroup>

 Just get confirmed
 $ sudo whoami
 ..[Password for <userName>]

 U will see root. -- hence root permission also granted.