At the moment anyone who is interested can register to play the Halo Infinite Multiplayer Preview on 24th September, all you need to do is signup via the Halo Insider Program.
To do this simply go to the Halo Insider Program Website and click signup under the become a Halo insider section. You must however register your interest / complete your profile by September 13th to be eligible.
Are you still interested in Halo? if so will you be signing up?