To play co-op in The Ascent, you can do so in one of three ways, and you can play it from the very start straight away with friends. The first is while on the main menu, click Online Coop or Couch Coop. If you want to play over the internet, click Online, but click Couch if you want to play with someone in the same room.
You will then have the option to either Host a Saved Game or Host a New Game. If you host a saved game, this will continue on a saved file that already exists, such as a file if you are playing the title in single-player. Hosting a new game is starting from the very beginning.
Related: How to fast travel in The Ascent?
Up to four players can be in one game. You can set the game as Invite Only, so the game is closed, but your friends can join via a game invite from yourself. Or you can set it to open, where anyone who has the game can join randomly.
The third way is that when playing on your own, by pressing the start button, you can change the Online Mode. This can be changed too, public, invite or off. Meaning, you can continue playing the game until a friend of yours comes online or random gamers join you, so you're not having to sit and wait around in a lobby waiting.
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