This is something that is definitely on the product roadmap (no ETA at this stage). We're planning to add the ability to add multiple customers against an incident, as well as whole companies and all customers of a service.
This will allow you to quickly communicate things such as P1 incidents, and avoid duplication of incidents where customers are having the same issue.
In the meantime, what you can do is, turn on "External CC" under Admin > Account Wide Settings. This will allow you to add external email addresses to the watchlist
Hope this helps in the meantime! :)
