as a quick idea, I would implement it via a Microflow set as “home page” for the respective user role in the navigation.
Here a sample microflow:
Another way to do it would be to have a container with conditional visibility with a message in it.
then make the conditional visibility based on user role.
This wouldn’t trigger a microflow or a popup, but might be a bit more relaxed. I don’t think triggering a popup when you log in would give you the best user experience :D