When a post is being assigned
If you are assigning a post, you can leave a first comment directly when you assign the post in the comment box, this is the one that the assigned user will see when they receive the notification
To see how to assign a post check this article
When a post hasn't been assigned
You can leave a comment at any time on a post by going to the comments tab.
It is important to note that sending comments is instantaneous, as soon as you press send, other users will be notified and able to see it. All the users assigned to the post, or those that have commented will be notified.
However, if a user is already in the comment section, they will need to reload the page in order to see it.