There's not really any requirement for a game to be included though generally games should 'complete'. That doesn't mean long and perfectly polished, more that it should feel like part of an anthology and not just a demo.
Submissions are as simple as linking the game(s) to the jam page I have set up on itch. There will always be a submission page open and indiepocalypse.com/submit will always redirect to the current active submission page.
As far as contributors go, they are chosen on a 'per developer' basis not per game. So while some people submit multiple games that end up getting included, there are always 10 unique developers each month.