For notification add H<groupname> and for Visual Voice add V<groupnumber> to a users Source Numbers.
No password required for visual voice mail access.
Pressing the VM butoon will show own vm box at the first line, scrolling down will show the group mail boxes (depending of the phone type, this is for the 1608/1616 phones)
It will show the group name and x/y/z were x = new messages, y = read messages and z = saved messages.
You can add multiple groups in the source codes but I don't know the maximum of that.