Google-groups – How to register for email replies to existing Google Groups posts

You can post a message to a closed thread thus:

1. log in with a subscribed account
2. click the 'forward' button on the post in the conversation that you want to reply to
3. put the group email address as the recipient
4. change 'FW: ' to 'RE: ' at the start of the subject line
5. write your reply and send

Source: http://groups.google.com/group/google-web-toolkit/msg/c3e3e6f250f4dcd6

As a bit of a spoiler, we tested this last night. I have two email address (which I will call for the sake of example "test1@gmail.com" and "test2@gmail.com"), and then we did this:

To:      Rebecca <Recip@gmail.com>
From:    My Name <test1@gmail.com>
Subject: this is a test
Body:  blah blah whatever

and additionally

To:      Rebecca <Recip@gmail.com>
From:    My Name <test1@gmail.com>
Subject: this is a test
Body:  this should nest under the other

and voila, they nested. No work required. And then we did this:

To:      Rebecca <Recip@gmail.com>
From:    My Name <test2@gmail.com>
Subject: this is a test
Body:  does this nest with the others?

And it didn't nest!

So in other words, two emails from the same sender with the same subject will nest. Two emails from two different addresses with the same subject will not nest.