You have an Azure subscription that contains the following users in an Azure Active Directory tenant named contoso.onmicrosoft.com:
User1 creates a new Azure Active Directory tenant named external.contoso.onmicrosoft.com.
You need to create new user accounts in external.contoso.onmicrosoft.com.
Solution: You instruct User3 to create the user accounts.
Does that meet the goal?
Only a global administrator can add users to this tenant.