Once you’re visible online and running the game, go into the Steam Overlay (shift+tab) and click on the Remote Play Together button. This will bring up an overlay box where you can send invites, adjust what inputs they have access to, and change what kind of quality the stream prioritizes.
The additional benefit of Remote Play is that you can allow your friends to interact with their own inputs. This means you can effectively host a local co-op game online, and their controller will act as a second player right next to you.
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