Final answer:
To facilitate real-time whiteboard collaboration for both internal employees and external users, Microsoft Teams should be configured, as it includes Microsoft Whiteboard integration for such purposes.
Step-by-step explanation:
For collaborative project work that involves real-time interaction on a whiteboard by both employees and external users, the best Office 365 product to configure in the Microsoft 365 admin portal is Microsoft Teams. Microsoft Teams is a robust collaboration platform that integrates various tools for communication, including chat, video conferencing, file sharing, and task organizing. One of its features is Microsoft Whiteboard, which is integrated directly within Teams and allows for real-time whiteboard collaboration. This will enable the group of employees and external users to work together effectively on the same canvas, regardless of their location.