Which service allows for grouping up accounts to administer them as a single unit effectively?

AWS Organizations is the service that allows for grouping multiple AWS accounts to manage them as a single unit effectively. This service provides a centralized way to control billing, governance, and policy management across all your accounts, making it easier for organizations to manage their cloud infrastructure.

With AWS Organizations, you can create organizational units (OUs) to which you can apply policies and manage members account-wide. This structure facilitates easier management of multiple accounts, allowing the organization to enforce policies, such as service control policies (SCPs), which govern what services and actions can be used in each of the accounts.

In addition to policy management, AWS Organizations simplifies the process of consolidating billing, which can lead to cost savings by allowing organizations to benefit from volume discounts across multiple accounts. Overall, it enhances administrative efficiency and governance over a growing number of AWS accounts.