Marina,
I would like to give the main group members the opportunity to contact
the moderation subgroup, but I don't want them to be actually part of
the subgroup or read messages among moderators.
In the Message Policies section of the mod subgroup's settings, check "Allow Non Subscribers to Post
Posts by non-subscribers will be moderated, instead of rejected".
The two limitations are:
1) Non-members here means any non-member; there's no way to restrict it to just members of the primary group.
2) Messages from the primary group members are moderated when posting to the mod subgroup. This one might be considered an advantage, but I mention it because it is not optional.
Shal
Shal Farley