Sirius is quoted (
http://unbit.nl/2014/09/16/martti-malmi-on-bitcoin-and-identifi/) as saying:
Googling for p2p currency I found the recently published Bitcoin white paper, which was the only completely decentralized solution out there. After reading it, Ive e-mailed Satoshi and offered him my help with the project. I wrote some documentation, helped refactor bitcoin.org and set up the bitcointalk.org forums. I did some work on the Bitcoin code, submitted UI improvements and ported the Windows-only client to Linux.
Besides Hal Finney doing some testing in early 2009, it was basically just me and Satoshi working on Bitcoin, until Gavin Andresen joined in 2010.
He found the bitcoin paper on the cryptography mailing list. Hal and martti/sirius were there.
I don't recall much discussion on the cypherpunks list (which was pretty much defunct at the time), but hal was originally a regular there too. Most of the cryptography list at least read the cypherpunks list and v.v.