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I can play with my friend with a steam account?
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From Larian Studios' FAQ:

Cross-play between PC/Mac and iPad is supported.

Cross-play with console versions of the game (PS4, Xbox and Nintendo Switch) are not supported.

So that seems to indicate yes, you can play with your friend with a steam account. Keep in mind the maximum number of players is 4 if playing online, and 2 if playing local split-screen.