Adding an additional user as administrator to Windows Server
There might be a situation where you may need to add an additional user as Administrator to windows Server or Windows Client Machine. Here you do not want to reset the password of existing user. Here is a method to do.
1. Boot the server or Windows machine with a live CD or USB or ISO. (Like Ubuntu live).
Here I am using Ubuntu ISO.
2. While booting from ISO, select try Ubuntu.

3. Click Files and click Windows Drive from Devices. (You may need to mount the drive depending the version of ISO using)

4. Go to Windows -> System32 and rename Utilman.exe like Utilman.exe.bak
5. Copy the file cmd . exe to Utilman.exe
6. Now, Shutdown the server and remove ISO / CD / DVD.
7. Boot to Windows Normally.
8. While at Windows Logon screen, press Windows + U key.
9. Cmd . exe is opened as we renamed the cmd . exe to Utilman.exe. This command prompt is opened with System user permissions with highest previleges.
So you know how to create a User using a cmd. Still I am giving the procedure below:
net user test P@ssw0rd /add /passwordreq:yes

This command will add a user test with a password P@ssw0rd .
10. Now add this user to Administrators group.
net localgroup administrators test /add

That’s it! You are done. And do not forget to delete Utilman.exe and rename Utilman.exe.bak to Utilman.exe