Hi Michael , you can set categories to be auto-followed for new users. New users who join will automatically be following categories with this setting.
You can also retroactively apply this setting to all existing users. Send us a request to email@example.com with the list of categories and we will update it to apply to all existing users.
The default setting for users is to receive new topics posted in categories they follow. Users can change this in their notification preferences to also include replies in categories they follow. (Or if you would like to change the default for all of your users to include replies, send us a support request.)
I hope this helps. Thank you!