We can specify recipients of an Activity Notification on a Notification Rule Detail screen as per the following screenshot.
If we add a new custom column a ticket, we would need to send a notification to a Contact or Group specified to the custom filed.
This document explains how to add a new Object Contact.
In order to add a new Object Contact, please follow the below steps. On these steps, a custom field named "SubGroup" that refers to "grp (Group contacts)" added to cr table is used as an example.