Each individual request has a Client User and a Firm User that can be assigned to the request. Only users that are assigned to a request will receive notifications about that request: such as when a document is uploaded, the status changes, or when a comment is added.
You can assign Client and Firm users when you create the request, in the new request form (as shown in the image above). Or you can add them after the fact by editing the individual engagement from the request list (as shown in the image below).
Once this is changed it is saved immediately.