Have you checked the subgroup's permissions under its own Admin, Settings?
These are handled independently of the parent group's permissions, and if memory serves they are set to Groups.io defaults regardless of how the parent group may be set up.? You may need to "loosen things up" a bit in the subgroup settings.
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Brian
? ?Some questions don't have answers, which is a terribly difficult lesson to learn.
? ? ? ? ? ?~ Katharine Graham