From Home Screen
Next to the client's name any arrow that you see will allow you to chat with that particular client.
On Client Profile
Tap on the particular client that you are viewing their path for and right there on the screen, you can also chat with them with the chat feature in the upper navigation bar.
Where will my Clients respond?
Clients will be able to respond to your messages within the Ate app where they will be notified with a red dot on the notification icon.