Secondary group for mail users
If this option is set, you must enter the name of a unix group to which all users
with email access for virtual servers on this template will be added. If the group you specify does not exist, it will be automatically created as necessary.
This feature is most useful in conjunction with Usermin, which can limit visible
modules to those users who are members of some group. So you could make the Read
Mail module visible only to members of the group that you set here.