Because I was interested, I believe that I've found the beginning of Groupthink. When we relaunched Crosstalk, I linked that post and though I hadn't looked before, I'm pretty sure that I've found the Groupthink announcement.

Instead of embedding it, I've decided to link in case anyone wants to get nostalgic or talk about the post. In addition to the Groupthink announcement which was part of a major site redesign, here's a follow-up from the next day where they provided a place to keep talking about bugs and to test things out, plus a random Jezebel page from those two days and a NiemanLab story containing the memo, where Nick Denton announced his new form of citizen journalism.