if I understand it correctly, when a group member replies to a message from GIO, that reply is queued for all members of the group, ...
That depends on the group's settings, one of which is Reply To.
... because the GIO group is the "sender" of the message.
Not for that reason.
The From address is generally the member's address and that would normally control where an email Reply goes. However in groups that have their Reply To set to Group (which is the default) then Groups.io includes a Reply-To: field in the header of outbound messages directing replies to the group's posting address. Nearly all email interfaces will interpret that Reply-To field to override the From address.
if a recipient wants to respond only to the OP, they must use the (blue) GIO reply to sender function. is there any optionality on this?
If the group is a free, barter, or sale type of group, and customarily has postings such as your example, the group's Reply To can be set to Sender. This changes the default state of the Private button in the group's web page, and it also changes the way the Reply-To header field in outbound email messages is handled. This means that by default member's replies will go to the Sender regardless of whether they reply on the web site or by email.
Shal
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