Final answer:
To ensure organization-wide access to a Power BI App with minimal administrative effort, publish the app to the entire organization, which gives all Power BI users automatic access.
Step-by-step explanation:
The solution to making sure all users in your organization have access to a Power BI App with the lowest administrative effort is to publish the app to the entire organization. In Power BI, you can create an app workspace and then publish the app, choosing the option to deploy to the entire organization. This will automatically give access to the app for all Power BI users within the organization without the need for individual sharing or access management. If you have content that requires more controlled access, you could instead publish apps targeted to specific security groups, distribution lists, or Office 365 groups. However, since your request is for the lowest administrative effort, publishing to the entire organization is the most straightforward option.