I am running Virtualmin 3.87 GPL on Debian Squeeze.
In System Settings -> Server Templates -> Template -> Administration User you can set the "Default Unix group for domain owners".
This setting is ignored.
I tested it repeatedly by changing the setting to a group (that definitely exists) and creating a new Host.
Yet at creation I still get the log message "creating new admin group x". Adding user to group ....
The secondary group works, but the primary is ignored.