At the end of each message (or digest entry) there should be a "Reply to Sender" link for the sender of that message.  That will trigger your e-mail client to open a compose window with that sender's address already prepopulated.

Just like there is a group message archive, there is a group directory for those members who allow their e-mail address to appear in it:  https://nvda.groups.io/g/nvda/directory

Since there are over 1000 members of this group, the directory spans 17 pages of content and, I just noticed (as I've never used it) there is no search mechanism for the member directory.

If you happen to know some specific keywords of a message a certain member posted, you can also search the group archive (https://nvda.groups.io/g/nvda/topics)
for a message using those, and a link to that member's profile, and e-mail button, will be part of the message itself, presented as a link at the very start of the message.
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Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763  

Growth for the sake of growth is the ideology of the cancer cell.

           ~ Edward Abbey