Hi Penny Chang , thanks for writing in!
Users only receive email notifications for categories and topics they are following. To follow a category, a user clicks the "Follow" link on the category page as shown here. (This requires that the Enable category following setting is enabled in the category.)
Or you can configure a category to be automatically followed by all users when they join. To configure this for a category, go to Admin > Categories and select the category. Under Notifications select New users automatically follow, then click Save.
Note that this setting only applies to new users who join. If you would like to retroactively apply this to all existing users, send a support request to firstname.lastname@example.org with the category name and we will set it for you.
I hope that helps to clarify. Thank you!