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what I was hoping was that if they were in the sub group when they sent the message, it would only go to the sub group

And it will, as Patrick noted the address of the email determines where it goes.? If they were in the main group and sent a message it would only post to the main group, if they were in sugroupXYZ and sent a message it would only post to?sugroupXYZ.

Of course that only works if one is using the website, if one is sending email there's nothing stopping them from "cross-posting", i.e. including both sugroupXYZ and the main group in the TO: field.?

Cheers,
Christos