You can¡¯t have subgroup members who are not part of the main group. If they are not a member already, they are automatically made a member of the main group when added to the subgroup.
If you have a small main group, you can make them the subgroup (and go no mail or special notices only for mail from the main group if they wish) and then add the new members to the main group.
I don¡¯t know of any other way you can keep things confidential
Frances
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