It is hosted on Slack... I believe Tom sends out an invite a day or so before.
If it is the same channel as last year, the link will be here: https://arbacfaauction.slack.com/
Dennis- I think you were in the auction last year, so you should be able to go to slack.com, click sign in and use the 'I don't remember my workspace' function, which sends you an email of slack channels you are subscribed to (that's how I found the link).