The key information here is "only five users" and "Agency users must have their original email address. You have added the agency's domain as a secondary domain."

For a small number of users (five), manually creating them in the Admin console is the most straightforward and least complex method. When creating a new user, the Admin console allows you to select the domain for their primary email address from any secondary domains you have added to your Google Workspace account.

Here's why the other options are less suitable:

A. Use the Directory API to automatically create the user accounts. While the Directory API can be used for automation, it requires scripting or programming knowledge. For just five users, this is overkill and introduces unnecessary complexity.

C. Use Google Cloud Directory Sync (GCDS) to sync users from an existing directory. GCDS is designed for syncing large numbers of users and groups from an on-premise directory (like Active Directory) to Google Workspace. For only five users, and if there isn't an existing directory that needs ongoing synchronization, GCDS is far too complex and unnecessary.

D. Bulk upload all users using a CSV file. Bulk upload using a CSV file is efficient for a larger number of users (e.g., dozens, hundreds, or thousands). For only five users, preparing a CSV file might take as much or more time than simply creating them one by one through the graphical interface, especially if it's a one-time task.

References from Google Workspace Administrator:

Add users one by one: This method is explicitly recommended for adding a small number of users (e.g., 10 or fewer). During the user creation process, you have the option to choose the domain for the user's primary email address from your available domains.