I just did this for my family. These are the steps I took. Everyone already has a steam account if that helps. I bought the game, and there's a section under my account where I was able to send invites to my family. I had each of them create an account on Eco, sent them an email via my account page (there's a section to do this from). When they then checked their Eco account pages they were able to download the game and then link the game to their Steam account all from their Eco account page. Does that make sense?
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RE: Steam keys