When a member displays a project, they automatically see the changes that have been made in their absence: in the What view, the pictograms corresponding to the modified elements change color (they take on the color of the project). The same is true for the dates of the tasks in the What, When and Who views.
Additionally, project members can subscribe to automated notifications. To learn more about this feature, please click here.
Finally, if you want to alert a member of changes made, you can send them a Buzz.