Super easy!
You already know the answer to this. A simple growth hack that should get you more than 100users is by either creating or joining a meetup or group with these interests. Give them some swag (tees/stickers)
If possible add gamification to your efforts and watch your user base grow ;)

If you want to chat a bit more give me or any one of us here a call thr can help you with growth tactics!

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