The only way we know how to message our client through business App is to message their Account through partner center - that way we pop up as "Your Expert" for them, and they pop up as "Account" for us. Furthermore, if someone on our team messages a user within that account (thinking.. @JohnSmith, @SallySmith, etc) only they would be notified, rather than the 20+ users that are on their account that they don't need to be privvy/alerted every time a message is sent in.