In your Azure/Office 365 tenant you may have several separate directories. You may also have custom domains but they will initially belong to a specific Azure AD.
An Azure AD directory is identified by the *.onmicrosoft.com
Here is how you create multiple Azure AD directories in the same tenant:
- Navigate to Azure AD Blade
- Click Create new directory
- Fill in information and select the correct country
- Click create and wait a bit
- Then click to manage your new directory
- Add custom domains if you need or just sync your new domain with dirsync
You may now add users or set up a separate domain sync. Or even custom domains.
This user catalog is separate and users need to be invited or added. Only one user will be present and that is the admin user that you were logged on with.