All users have the ability to be notified about the following:
When they are added as a member to a Group Library
When someone is added to one of the Group Libraries they are a part of
When a new Gem is added to a Group Library they are a part of
When a new Gem is added to the Organization shared library
When a user comments on a Gem they created
When a user adds a Tag to a Gem they created
When a Badge is added to a Gem they have access to
When a Gem they have access to was rated
Each user can change their notification setting under their User Profile > Notifications.