As Dusk Falls does support crossplay, and you can play with friends no matter which platform you are all are on.
Is crossplay supported in As Dusk Falls?
The developer for As Dusk Falls, INTERIOR/NIGHT, has confirmed that the game supports crossplay capabilities. They even retweeted a Tweet that states as much. There should not be any issues with multiplayer across different platforms, since the functionality has been factored in while the game was in development. As Dusk Falls is currently exclusive to Microsoft and Steam, meaning you'll have those options for crossplay multiplayer.
Can you play As Dusk Falls with a friend on a different platform?
It does not matter if you have As Dusk Falls on Xbox and your friend has As Dusk Falls on Steam or vice versa. Either way, you will be able to play multiplayer together. INTERIOR/NIGHT, the developer of As Dusk Falls, has stated numerous times that crossplay is in the game. Don't worry about whether or not your friend has the console or PC version, the functionality will not change. The only difference may be what buttons or keys either of you will use.
Want to read more about As Dusk Falls? If so, check out How to play As Dusk Falls with friends on Pro Game Guides.