As for marketing, I met learn how to appeal to communities, improve your sales pitch. And considering it's a free game, that should be easy! Things like implying scarcity, or urging action through time limitations.

c/gaming: Now Accepting Beta Testers (indie dev, limited spots fill quickly! See details below)

If you're interested, sign up quickly! I'm only accepting a limited amount of beta testers for (elevator pitch of game, brief description of yourself and your history to appeal to emotion).

If you want to have your voice heard in game development, be a part of the journey of [game], or just want access before anyone else, apply now!