You could go to Admin panel -> Settings -> Alerts and Notices and set New Message Alert to use "Last Respondent" and/or "Assigned Agent/Team" but I dont think that will work.
This doesn't help, I already made this setting.
You can add them as an account manager to the organization... that might work.
No he allready is an account manager and that doesn't work.
How about adding this function in one of the next versions?