In order to receive email notifications for certain events you will need to first set up email notifications.
You sub-admin will need to enable email notifications for the subscription. Then each user who wishes to receive them will need to edit their own user profile and opt for that option.
The most straight forward way to test that you are receiving email notifications is to log out of Agile Central, then click Forgot Password and type your username. This will send you an email at your registered email address on your user profile. If you did not receive that email then you may want to find and correct that issue before proceeding. You are not required to actually change your password, it is only used as a way to confirm the emails are sent and received.
More info on setting up email notifications is at: https://help.rallydev.com/set-your-notifications
How to receive an email notification every time a discussion is added or modified for an artifact ?