Project managers can subscribe other users to a set of items or selected items to be notified of future changes.  In a set or folder, click on the corresponding button at the top and go to “Subscribe Others”.

On this window, select the user or user group you wish to subscribe to the item, set or folder.  Users will receive any updates to this item unless they mute themselves (see: Managing Your User Profile).  By default, users are set to immediate notifications, but this can also be changed in their user profile.