Yes, you can grant admin privileges to users from an external organization (such as your marketing agency) without needing to create a new user account in your own Google Workspace, which would require an additional license. You can do this by using the "Admin role delegation" feature with Google Workspace domain sharing.

Here are the steps to delegate admin access to an external user without creating a new user:

1. Set Up Trusted Domain Sharing (Optional)

2. Delegate Admin Roles to the External User

3. Grant API and Services Access (Optional)

4. Review Security and Permissions

5. Monitor Activity and Access

By doing this, you avoid paying for an additional Google Workspace license and ensure the marketing agency can manage your account using their existing credentials.