Halo: Reach will be joining the Master Chief Collection on PC and Xbox this Tuesday, December 3rd. On PC, it will be available via Steam and the Microsoft Store. The game will be available to play at 10AM PST/1PM EST. You can pre-download it on Xbox and from the Microsoft Store, but not on Steam. Reminder that players on PC will be able to play with one another regardless of where the title was purchased.
EDIT: Ske7ch has posted on Twitter that pre-loading will not be supported. https://twitter.com/ske7ch/status/1201556645286793216