Linux in hand

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.
 
 support -- https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux_OpenStack_Platform/2/html/Getting_Started_Guide/ch02s03.html
 
 To change the password of a user
 1. switch as root user 
 sudo su -
 2. passwd <userName>
 3. Provide the new password for the same.