This was just discussed in another thread. PM is not yet a feature in groups.io.
You can go to your members list and click on either "Display name" at the top to sort the list by name, or on "email" to sort the list by email address, and find their email address that way. Or you can go to one of their posts and create a private reply by clicking on "reply" and then on "private" at the bottom right. Or you can try to find an email from them in your inbox.
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J
