Regarding collaboration with external team members outside the own organization.
In one use case, communication is expected to be highly delayed, e.g. for tasks that have a considerable time gap. The way of working there is more like a web forum.
Not every team member will have a Webex license, nor is it expected that everyone will install the app. I expect that some team members will connect via a web browser.
An email notification would be very helpful to inform colleagues of new activities/messages.
This feature seems not to be implemented yet.