I got a invite but have no clue how to download it on my console
Outside of downloading the Halo Insider app, there is no news on this. They are fixing some technical issues they found during the stream and the flight has been pushed back.
https://www.halowaypoint.com/en-us/forums/243780d3b80b4bb6ac7917a8942286db/topics/psa-halo-technical-preview-2-updates/511173a1-85aa-4941-b02f-12645a4917ea/posts
The email literally says that you will get instructions closer to the launch of the flight. Just wait.