Unity 400 default root password

As the others have stated, root is not available except via support channels. You shouldn't need to use it, unless its specifically required.

If you want to create users via the CLI, you'll need to install the UEMCLI Client for your environment.

User guide for the UEMCLI is here:
https://support.emc.com/docu69330_Dell-EMC-Unity-Unisphere-CLI-User-Guide.pdf?language=en_US

This assumes that you are using an account that has the correct privileges to create users.

uemcli -d {array ip} -u {local or fqdn/user} - /user/account create -name {value} -role {roletype} -type {local | ldapuser} -passwd {password} or -passwdSecure

e.g uemcli -d 10.0.0.1 -u doman.local/admin -p {password} /user/account create -name 'TestUser' -role operator -type local -passwd Password123!

To see the list of User Accounts on the Array: 

uemcli -d 10.0.0.1 -u doman.local/admin -p {password} /user/account show

This should show the account in the list of Users